A few years ago, I was visiting a friend in San Fransisco, and it was a warm, sunny day (to my friend's surprise). We were hanging out at a beach when we see a guy in a speedo walk by, towards the water. Before anyone could warn him, he jumped in the water, immediately jumped out, then ran back to the parking lot.

My guess is he was European - people from the West coast would know not to go in the water, and American men rarely wear speedos. I imagine the concept of being at the beach on a warm sunny day, but the water is too cold to enter, was completely unknown to him.