Depends - on the weather, sea streams.
Usually it's around 20 Celsius degree. But this year - I've heard - it's colder and it's hardly a pleasure to go to water. Even though the weather has been really hot for a couple of weeks.
sorta had a crush on a Polish girl once...
Depends on the sea current - water is sometimes cold, but I've been to Hel a week ago and there was a lot of people on the beaches!