We have this saying in Finnish, heittää talviturkki pois, which roughly translates to throw away your winter fur. It means to go swimming for the first time after the winter, and I threw mine away today!
I googled and the water temperature in the lake was about 15 degrees Celsius, air temp hovering around 28 degrees. It was soooo hot today, that I really needed to go swimming, and boy did it feel great! After I whined at the knee deep water for 5 minutes before plunging in.