I don't know the water temp. I'd say it's probably somewhere between 12-15 degrees - by August it might reach as high as 17. But, you know. That's a guess, really.

And depends on what you mean by 'common': I've been doing this since I was very young, as has my mother, but it does not appear to be an exceedingly popular pasttime among the population at large. Some sections of the coastal-dwelling populace, yes, but not so much everyone else. Unless there is a heatwave in June or July. :)

(Swim early, before the jellyfish descend. Because they keep coming earlier every year.)