Thanks Elsa, Golunvolo and Simato.
Indeed there are a number of challenges shooting underwater, as highlighted by Simato. One would have to be a competent to good scuba diver having the necessary skills to control buoyancy, breathing, coupled with photography skills, and a rich UW environment.
Close-up of extremely tiny marine creatures presents an enormous challenge - as most of you would appreciate, macro is first of all quite tough without the use of a tripod, plus the fairly low lighting levels UW, and the tissue of breathing changing the diver's buoyancy and cause movements ...
Marine waterscapes, as it were, would pose the challenge of balancing artificial lighting (strobes) and natural lighting, photographing in moving currents at times.
I suppose simato you might prefer these:


