I had sometimes swam in relatively cold water around 10 degrees celsius (50 fahrenheit) but a few weeks ago I jumped in the ocean when the water was 0C (32F). Since then I have been kind of hooked going once or twice a week (after workouts) and it’s incredibly invigorating. I’ve never experienced anything like it really.
At first it’s always a shock to the body but I try stay in for a few minutes (always keeping head above water and never going too far out). And the body adapts to it, it gets easier each time.