It's some sort of worm that swims around the water. And it actually kind of freaked me out quite a bit. It was just creepy. The bottom picture is the scaled down shot, and I have included a full-sized picture of just the worm. (Kind of funny - told someone at the dive store that if I never see another one, that'll be fine with me - and the following night at Day Island I saw another one. Yuck.)