For the South/South East, very warm water temperature, if we get them (which we haven’t really the last 2 summers), would be 17°C/18°C i.e. 62.5°F/64°F. Or at least there were 4/5 days I recall last year that I swam here in those temps. There was no consistent period however at that level, except one good week in May 2009.
But the last two summers here have been dreadful. We would have reached those summer temperatures regularly before. The SW wind is slightly warmer, maybe 1deg C, because it catches the Gulf Stream better. All around the North & East coast could be 2 deg C colder. Best regular water temperatures last year during August/Early Sept. were around 14°C/57°F in Tramore Bay, Waterford where I mainly swim. I often swim Sandycove in Cork more towards the SW, (one of the world’s great swim locations) and sometimes find the temp. can be a degree warmer.
Typical south coast Winter water temp might drop down to 8°C/46°C, with 7°C/46°F common in the SE. That can vary a bit depending on wind direction again and occasionally rising cold currents.
The winter of 2009 though, after the New Year, saw apparently the coldest water in 10 years with one day at my main training spot measuring 4°C or 39° F. Certainly since I started sea swimming 4 years ago and being a surfer before that, it’s hard to say,a since I was always wearing a wetsuit back then.
Oh, yes, I almost forgot. Typical tidal range on the South coast is 5 to 6 metres. Gotta remember that for a lot of swims ’cause it means strong tidal currents at times and locations. Spring tides can get up to 7 metres.
I’ll do a long article on local and south coast tides soon.