For anyone starting out in underwater photography, close-ups of soft corals is easy and visually rewarding.
These shots are from a film camera. Digital makes it even easier. Some people spend most of their underwater lives looking at and photographing things no larger than a coin. Others enjoy making a library of fish pictures.
The most boring (and therefore the easiest) subject of all, is another diver with a camera pointed at you.
If there is a shark or something exciting between both of you, that is a different matter!