Underwater photography is a technique specifically difficult to master. As in all other photography genres, there are some parameters to take into account in order to create a nice picture (even if a good technician is not enough to produce great photos).
More than in any other photography types, you have to adapt to a very harsh environment: Limited light or no light, density of the medium, incompatibility between water, pressure and mechanical or electrical hardware, optical characteristics of water.
This is why it is often necessary to acquire a number of core competencies before becoming able to produce nice images. I admire all the more those divers who invest a large part of their hobby time (or their professional time) into underwater photography.
For those willing to try or develop this technique, I would recommend heartily a web site which I found in Photography Blog: Dive Photo Guide. Hardware informations, techniques, tips and tricks, tutorials, ideas; All you may need for underwater photography for photo-divers.