The North Sea near the coasts is around 4°C right now, but in warm winters it can get warmer, and in summer it reaching around 18°C (so around 64°F?) is fairly normal, sometimes 20°C even. But 20°C water still doesn't feel warm, and I assume you can get hypothermia if you stay in for a long time or something and aren't trained in cold water swimming?

I mean, obviously psome eople swim in very cold water and people can train to withstand stuff to some degree, but for bathing in the sea as a normal person, I've been told the rule of thumb that without protective gear you can stay only as many minutes as the water is in Celsius, or you'll get too cold, even if the water is at summer temperatures.