I have a lot of difficulty taking pictures of fish. The Canon Elph camera that I use has a noticeable auto-focus delay, aggravated by the lower light conditions under water. The fish have a tendency to move away during the few seconds it takes me to get a focus lock on them, so many of the fish pictures are a little blurry. Coral, on the other hand, are easy to take a picture of -- then tend not to move around.
Parrotfish are particularly hard. The sight of a snorkeler causes the parrotfish to turn and slowly swim away. I often try to give chase but often end up with just a blurry picture of the parrotfish's tail.