Ya know, it`s just being in the water. Maybe working on skills. If you can be comfortable and stay with your buddy in a murky lake then a tropical reef is a piece of cake. And there is the topside comraderie that goes with a day at the lake.
Plus, there are fish and plants and sunken objects like boats to look at. But ultimately, you either see the appeal or you don`t. I don`t have much use for cycling but lots of people do.