Those who don’t swim in the swim much are often confused about what temperature to expect.
The specific heat capacity of the ocean is greater than land or the atmosphere.
This mean it’s will stay at a specific temperature and not quickly change.
The volume of the ocean contributes to it retaining heat, as only a much smaller volume of land can retain heat.
Short-term atmospheric conditions have little effect, so for example, the extremely cold spell over Christmas and the New Year had little effect. The primary cause of temperatures changing are ocean currents.
My own notes from last year the water temperature is about the same. Christmas Day, coming after almost two full weeks of very cold temps was still 7.5 Dec Celsius. I measured that same temperature last weekend, toward the end of January, after the even longer cold spell.
Ocean temperature also doesn’t vary much over a 24 hour period, less than 1 degree. So for those yet to experience a night-swim, because actual air temperature can drop, but the sea stays the same, the water may feel warmer at night, (though it is about the same).
Because it takes much longer to change, the larger implication is that the ocean temperatures “lag” the seasons. It’s warmest in autumn after the summer is over usually September or so, and coldest in February/March, usually after the worst weather has broken. The coldest day I measured at Tramore in 2009 was 5.1 Deg. Celsius on the last weekend of February.
I find it can stay fairly comfortable to late November, certainly still above 10 Deg. Celsius.
There are of course local variations. Sandycove has that coldest bit right at the entry because of the fresh water inflow. T-Bay has a shifting (but very rare) up-welling cold current, that can drop the water temperature by 2 degrees in 24 hours.
Summer rain run-off, streams, rivers , all have local effects. But by local I mean maybe 100 metres at most.