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Edit: this post, the site’s most popular, has an updated version.

This post is courtesy of searches on the site as a few variations of this question have cropped up.

I guess one could divide thoughts on lowest possible water temperature in which to swim into three camps.

• 1 degree WARMER than it is now
• What it is now
• 1 degree COLDER than it is now

Substitute any temperature reading into the above sentences…because cold is fairly subjective, (up to a certain point). I used to be in the first category, moved to the second, and am probably now is the third. (All this means is I’ve swam in 5 C. which makes me think it’s possible for me to swim in 4 C. It’s a moving target).

I’ve pointed out before some of the things that affect your ability to deal with cold. Let’s try and make a more comprehensive list.

• Will you be wearing a wetsuit?
• Are you wearing a swim-cap?
• What height are you?
• What weight are you?
• What shape are you?
• How did you sleep last night?
• Are you tired just before you swim?
• Have you drank alcohol in the last 24 hours?
• Have you eaten (properly) today?
• Are you well or ill?
• Or have you been ill recently?
• Have you swam in similar temperatures before?
• If so, for how long?
• If so, how often?
• Does Open Water scare you (just be honest with yourself)?
• How well do you know the location?
• Are you cold before you swim?
• Is it sunny or cloudy?
• What’s the air temperature?
• What’s the wind direction?
• What’s the wind speed?
• Is it choppy or calm?

So, as you can see, there are lots of variations just with these parameters. Some, like illness, are less likely but you really need to be aware of your own experience and take it incrementally.
One can’t reasonably expect to go from pool swimming to doing an hour in 7C / 45 F without a wetsuit, based on desire to swim alone. Granted, this isn’t likely to occur, but I’m trying to illustrate a point.
Ability to handle COLD is again a matter of a few factors more important than others (all other things like alcohol, food, illness, sleep being equal): namely, experience and weight.
People with plenty of experience of cold can swim in very cold water. I can swim for 20 minutes in 5 C / 40 F water, because I’ve gotten used to it. But I certainly don’t recommend it and I won’t claim it’s fun. And the bigger and heavier you are the more you can handle with less training. Fat is an insulator. Just having plenty of fat alone makes cold easier to deal with. But fat does not lessen the pain of the initial shock for example.
Finbarr makes a comment that is highly relevant also, that I should have included and that is the effect of wind. Any Northerly wind in Ireland is inevitably cold. Heat will be stripped from your body faster while swimming and while trying to get dressed. (Easterly winds may also be cold). Any wind will generally cool you faster. And there is no thinking your way out of it. A similar effect is whether there is sunshine or not. The day of the Guillamenes video below was flat calm, no wind, warm air and sunny. I think I swam about 50 minutes that day, and even thought the water was no warmer than now, I felt much more comfortable, due to the lack of wind combined with direct sunshine and calm water. I’ve said before, wind is the swimmer’s enemy.

I can also tell you, without any embellishment, that my reactions to various temperatures are entirely different now than they were two years ago. I wrote a chart for myself of my reactions and estimated comfortable swim times at decreasing temperatures below 12 Celsius. That chart is now entirely useless as a current indicator, but is interesting to me as an measurement of how my ability to handle cold has improved.

Being a man, I’m completely unqualified to comment on the effects of cold on pregnant women, sorry. Normal “seek appropriate medical advice” caveats and warnings apply.

I have done some reading on regular cold water immersion. It seems the evidence says regular immersion in water temperatures of less than 10 Celsius is very beneficial for health, in a few different areas; improved respiration and circulation, lessened chances of infection and heart attack. However once the time goes over 10 minutes some of those benefits tend to reverse, especially hypertension and cardiac arrhythmia.