One day when becalmed half way from Hawaii to San Francisco I jumped overboard for a swim. Of course the sails were down and boarding ladder down and a floating line in the water. It was something to look down below my feet and think of the 18,000 feet of water below me as I floated at the surface.
I had a mask on, and took a few snaps with a primitive underwater camera. There was nothing to see. No small fish and nothing but blue and light rays from below (an illusion). But with sharks (the worry of everyone) it is the ones you don't see that are most likely to get you.