If you do decide to go swimming in the ocean at night...my wife and I used to do it all the time at our resort. Stopped abruptly one night when a friend and I decided to go for a swim and something had happened to the lighting and there was a shadow over the area where I decided to enter the water. I went on in and it was like entering an acid bath. Next morning there were tons of baby jellyfish washed up on the beach. So, look before you leap...my experience can be filed under "young and stupid".