I don't really like swimming in natural bodies of water--they too often contain creatures that can eat or sting me, and it's too hard to see said creatures through the murky water. If I do end up at some beach, I prefer not to go in over my head--the only reason I ever do is to water ski, which is something you really can't do in a swimming pool. I've never had any sort of large-water-body accident--short of tumbling off the skis and landing on my face or ass--but still, deep water creeps me out, and I remember that each and every boat trip, I was anxious the first few times I jumped off the boat so I could bob along on my butt before being dragged out of the water by a rope.
Beaches often pose another problem you don't face at a swimming pool, namely, a lack of dedicated places to pee. So what's the best solution, environmentally speaking, when you're at the beach with a bladder that must be emptied? Mercifully there's a website that will answer that question for you.