When I turned 30, I decided to mark the life milestone with a plunge in the Pacific. It was freezing. Absolutely numbingly freezing. The cold cut into my skin like tiny pins, and it literally pinched my lungs so I couldn't breath. I was in only for a few moments, but, stepping out, I have never felt so alive.
PS--This was the Oregon Coast, not a nude beach. It was mostly vacant, being winter. But none of the other visitors seemed to mind my birthday plunge.