How To: Understand the Features and Hazards of the Different Types of Open Water Swimming Locations – I – The Ocean


Open water is open water, right?

Well, yes and no.  Sure, water itself is only water, whether fresh, salt water or brackish, and if you can swim, then theoretically you may think that’s all you need to know.

Unfortunately, too many beginner and intermediate open water swimmers don’t think about the various conditions and hazards associated with different types of open water locations. I remain interested in the huge swathes of knowledge and experience about open water that few people communicate and that so many of us find ourselves learning from scratch. Even the best online swim coaches spend almost all their time talking about stroke and technique. All other aspects of open water swimming are often neglected.

Being responsible for your own open water swimming safety means you should be able to understand at a glance the main characteristics of a particular location as you build up your experience. “Just swimming” applies more to the pool than it does to the world of open water, where variation, change and the unexpected are the very features that we embrace about open water and that make it continuously challenging and interesting. If you climbed one mountain today and then next week decided to climb another in a different location you wouldn’t automatically think that all mountains are the same and give no thought to the ascent. At the least a similar degree of wariness is required for the ocean.

While we may be familiar with one type of open water swimming, not all have the same hazards and we need to be careful not to be complacent or extrapolate incorrectly from one type of location to a different type.

So let’s look at the different locations and their main challenges in this two-part series.

Open water swimmers whose background is lakes or rivers can easily underestimate the huge range of possibilities present in and presented by the ocean and even elite pool swimmers will flounder on their first day in the big blue if conditions aren’t ideal.



  • Unpredictability. “Never turn your back on the ocean” is an old adage that clearly conveys the unpredictable nature of the sea. Please see this previous post on the unpredictable nature of waves on the coast. Every open swimmer regardless of experience should treat each swim in the ocean with respect and fore-thought. Always expect the unexpected and swim within your own bounds, because if you try to swim within the bounds of the ocean, it will always surprise and inevitably defeat you.
  • Global ocean salinity

    Global ocean salinity

    Salty. Average global salinity is around 4% but this varies quite significantly by location. A local 5% salinity actually means the salinity you notice is 20% greater than the average of 4%. Pacific Ocean open water swimmers used to 3% will notice that Atlantic around Ireland is noticeably saltier, while Irish Atlantic open water swimmers will notice that the English Channel at 5% is noticeably saltier again.

  • Buoyant. Salt increase water density, which makes you float higher, which makes you less tired and able to swim faster and further. It can also make you over-estimate your capabilities. The previous differences in salinity are the factor that determines buoyancy.
  • Tidal. Tides vary hugely by location from zero tide (which points are called amphidromes and occur around the world) to 15 metres (Bay of Fundy, Newfoundland). Southern California and the Mediterranean are less than a metre, whereas Ireland is about 4 metres height and the English Channel is over 6 metres.
  • Wildlife. Fish, jellyfish, plants. Some bite, some sting, most are harmless and all should be left alone and some will look at you malevolently.
  • Greater depth. While there are some very deep lakes even those pale in comparison the ocean’s depths. Depth doesn’t affect a swimmer’s ability but adjusting to the void beneath you can be psychologically difficult for some.Wind & Waves
  • Wind and Waves. I’ve categorized these two together as waves are caused by winds (excluding tsunamis). The further the distance that wind can blow over a surface the greater the waves that will result. So you can strong wind blowing over off the coast with no waves. The stronger a wind the greater the resulting waves. You can have wind without waves (locally, if for example the wind is blowing offshore and you are near to the shore) and waves without wind (powerful groundswell travelling from along way away) but waves are a visible tangible manifestation of wind energy somewhere.
  • Changing temperature. Ocean temperature patterns change slowly because the volume is too large to do otherwise. Effects of this can be that the sea will colder on warm days in early summer than expected and warmer on cold days in early winter. The ocean is also prone to patches of varying temperature and even a variation of quarter of a degree will be noticeable to regular swimmer. There’s a significant amount of articles here about cold water swimming, which carries great risk than warm water swimming.



  • Weather. Experienced open water swimmers place present wind conditions above all other facets of weather. Fog, lightning storms, hailstones. I believe that an experienced and capable open water swimmer must be attuned to these facets.
  • Black and white photo of a sudden squall storm in Tramore Bay in Ireland

    A squall appears as if from nowhere over the east side of Tramore Bay, while the west nearby side is completely calm.

    Sudden weather changes. Any regular ocean-goer can tell you of almost unbelievably sudden and local weather changes. A squall is such a highly localised storm where winds can suddenly increase violently or shear. Squalls will not be captured in weather forecasts.

  • Changing visibility. Squalls, fog (sometimes considered  the most dangerous ocean condition for open water swimmers), mist and haze and even low sunlight on the horizon or glare off the water can all reduce visibility, and confuse direction and navigation.
  • Navigation. Finding your way is one of the most difficult open water facets to master, comprised as it is of two separate facets (navigation and swimming straight). Anyone who have never seen a coastline from one hundred metres may find it difficult to believe even coves and bay and features that are overwhelmingly obvious on land can disappear within such a short distance.
  • Unexpected waves. I’ve written about this previously in There’s No Such Thing as A Freak Wave. The oft-reported coastal drownings ascribed to freak waves are really describing normal ocean behaviour dictated by simple physics. If five, seven or nine waves are all roughly the same size, it does not mean that a subsequent wave which is twice the size is a freak, merely unexpected and difficult to predict. Also the idea that the Seventh or Ninth Wave is always larger is also a myth.
  • Tidal currents. A tidal current is not a rip current or an undertow. It’s a usually larger-scale current associated with the movement of the entire mass of water due to tide changes. However, like other currents, tidal current can also have stronger localized effects and will be generally be stronger where tides are of greater magnitude.
  • Other currents. Apart from the aforementioned Tidal Currents, there are other types of hazardous currents in the sea including:
    • Rip Currents. These localized currents are escape channels for breaking waves, and can be extremely strong, running directly or obliquely away from the shore.
    • Longshore Currents. These run parallel to the shore and can be longer than rip currents. They can also combine with rip currents.
    • Channel currents.  The general term for water flow compressed  into a stronger flow in a narrower space.
    • River Outflows. Rivers entering the sea can be quite strong depending on the river volume, width and fall height.
  • Wildlife. It’s all good because it’s all meant to be there. And as an open water swimmer, so are you. Your responsibility as an open water swimmer is to protect the natural environment of the sea. If you can’t do that, then it’s not the place for you. Some jellyfish sting, some don’t and some are lethal. Some fish bite, and some don’t. Ocean wildlife in general is less dangerous to you than your motorcar drive to the sea.
  • Depth. If you have only ever swum in shallow coastal water, the mere knowledge that you in water 100 metres or even kilometres deep can be psychologically difficult
  • Hazardous Exits. Underestimating the difficulty of safely exiting the water in windy conditions is a far more genuine concern for most than worry about sharks. Someone swimming in a location with greater tides than those with which they are familiar should consider how changing tide heights will affect their exit.  This is something you must plan before you consider entering the water. Half of tide’s entire height range changes in only two hours, meaning water level can change by a metre in many locations in only one hour. Though a safe location the Guillamene steps at mid tide demonstrate some of the height variation possible. At low spring tide the surface is at the bottom step of the ladder beneath the steps. At as high spring tide, all the steps even higher are covered.


  • Reduced visibility below water. Winds or estuaries or marine craft can raise sand and destroy your ability to see your hand directly in front of your face. This can lead to impacts with rocks. It’s not itself dangerous, but another feature. Lacking visibility is not itself a bad thing. Anyone who ever swims in Dover’s Harbour’s notoriously impenetrable water can instead focus on the susurrus of the shifting shingle.
  • Visibility from the shore. There’s an old story of my first ever non-wetsuit swim. I started in March before I starting swimming year round and didn’t realise I was starting at the coldest time of year, before I’d built up the experience I later developed. It was in a sheltered cove but I had been terrified about the cold and conditions were choppy. I’d planned a short swim but I was apparently invisible almost immediately and my partner thought I’d been swept away. Alternatively I am constantly worried that someone seeing me heading out to see will panic and call the Coast Guard, as has happened previously.
  • Pollution. Often very hard to know about. Local knowledge is useful, watching for livid green explosions of kelp or algal blooms on rocks under stream outlets, old pipe outlets and at its worst faecal masses, garbage mounds or masses of dead fish are all indicators. Sometimes there are none.  Marine diesel spills, slicks from boat exhausts, water run-off  from urban areas after storms and agricultural pollution are other examples. The only protection you might have is the dilution effect of the large volume of water, (and whatever you can do personally to address problems and causes).
  • Marine craft. I have always thought that any swimmer who spends a lot of energy worrying about sharks, has likely never been trapped under a boat or had a near miss with a propeller.
  • Jetskis. Because I hold these things in such contempt and loathing I refuse to honour them with classification with the rest of the marine craft on the ocean. Be afraid and wear a bright swim cap.
  • Theft. Yes, theft is universal. But popular recreation spots near the coast are particular targets. Using a Keypod or a Tow Float are alternatives to risking theft.
  • Shore anglers. This is a hazard at my usual location and caused the loss of one of the waterproof cameras I previously bought after someone decided to fish in the No Fishing designated swimming zone, and pulled a  new camera from my Speedoes to the bottom of Tramore Bay, where it likely still resides.
  • Rocks. I like rocks. I like playing around rocks in the water. I like when the triathlete in the race who thinks he can swim between me and rocks realises I won’t give way, and suddenly understands that my skin will grown back, but his rubber won’t. I use rocks as race allies and I am really comfortable swimming though reefs in rough water. But I also have a significant number of scars because rocks are sharp and hard, and often invisible and cold water makes bleeding invisible and numbs even large lacerations. Don’t, whatever you do, dive into unknown ocean as people die from cranial impacts on unseen rocks.
  • Seaweed. Some people are afraid of seaweed. I don’t get it, but then I have my own phobias that you probably won’t understand. Seaweed or kelp like dabberlocks at its thickest can make a stretch of water impassable.

I have over the years come to think as open water swimmers need to be more reflective about the many aspects of their obsession/hobby/lifestyle/whateverthis is. The world is replete with people who study every aspect of the sea. We ourselves should be experts in our field because by its nature, the sea is so much than any of us. Increasing knowledge is one of the most accessible ways of both protecting ourselves and allowing us to enjoy the sea more.


Related Articles:

How To: Understand Tides for Swimmers, Part 1 – Theory

How To: Understand Tides for Swimmers, Part 2 – Local Effects

HOW TO: Understanding Waves, for Swimmers, Part 1

HOW TO: Understanding Waves, for Swimmers, Part 2

HOW TO: Beaufort Wind Scale – An essential observational skill for the OW Swimmer

HOW TO understand Sea Area Forecasts / Shipping News

Sea Lice, Seabather’s Eruption and Swimmer’s Itch





11 thoughts on “How To: Understand the Features and Hazards of the Different Types of Open Water Swimming Locations – I – The Ocean

  1. I live in “Taiwan.” So reading your article is translated with Google, so …, I can only reply with Google translation. Thank you very much for learning your knowledge.


  2. Pingback: How To: Understand the Features and Hazards of the Different Types of Open Water Swimming Locations – III – Estuaries and Man-Made Locations Including Quarries, Reservoirs and Canals | LoneSwimmer

  3. Pingback: How To: Understand the Features and Hazards of the Different Types of Open Water Swimming Locations – II – Lakes and Rivers | LoneSwimmer

  4. Pingback: 13 Miles Completed @AspChannelSwim | pmfSwimming

  5. Hi Donal,

    Reminds me of that churning yellow mass that chased you from the Metal Man back to the Guillamene and is still one of the strangest sightings I have seen. Deep scattering layer activity out beyond the bay? I wish someone could explain what it was. I am wondering also if you have ever arrived for a swim and not necessarily due to weather have made the decision to not swim? Hope you are keeping well!


    • Hey Sam,

      Yes it has happened, but I do try to keep days like that to a minimum for myself, because I’m afraid each might be the thin end of wedge. But someday you are better off not swimming. It’s slightly more likely to happen me on the pool in fact, where a couple of times a year, I might get a kilometre done and give up because I’m completely wasting my time that day.


  6. As usual, a thorough analysis of the subject. A couple of personal stories to add to the mix:
    * Under “Pollution” (or maybe a new “Unexpected Objects” heading): when I swam around Manhattan Island, I had the experience of swimming through a batch of rotting cataloupe (melons). Running into anything strange or new whilst swimming in open water can be surprising, but having your hands slice through large, round, soft objects was particularly unnerving. Hard to prepare for that. LESSON: Count on the unexpected and leave it at that.
    * Under “Hazardous Exits”: I was once swept off the top of a stone jetty extending from the beach in Texas. Jetties there are made of “rip-rap” (large blocks of granite) jumbled together with sometimes a thin walkway of poured cement in the middle. After I had bounced my way into the water (acquiring numerous sea urchin spines in my flesh along the way), I had to figure out how to get to safety (the sea was very rough and I was at the end of a very long jetty). I initially thought I could time the wave action and scramble back up the granite blocks. My higher mind wisely determined that was a fool’s errand, so I just swam the long way to the beach, where my friends took me to a local (drunk) doctor who patched me up. LESSON: find the safest exit, even if it takes longer.
    * Under “Shore Anglers”: On one of those same Texas beaches, I was once hooked by a fisherman from a wooden pier as I swam by (apparently, I presented an irrisistable target for his casting skills). Once I had removed the hook from my heel, a vigorous argument complete with colorful insults ensued between the participants. I was advised by a friend to not continue the argument by rushing onto the pier. Again, my wiser mind prevailed, and I swam off. LESSON: stay far away from places where fishers are fishing. Remember that they have weapons and you probably don’t.

    Love your stuff, Donal!


    • Thanks Harald.

      My assumption when I posted it was that there was something I’d forget. It wasn’t as easy a list to write as I thought, it took two or three attempts to get it all down. You are of course right on all your stories.

      Your story of the watermelon reminds of Lynne Cox’s story about swimming in a marathon in the Nile in Cairo and instead of a watermelon…it was a dead dog! I can say I myself have swum into a empty plastic bag that scared the bejaysus out of me.


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