I get asked about taking photos underwater all the time. My biggest piece of advice for taking good underwater photos is to have good buoyancy control. To really improve your underwater photography I would recommend the Peak Performance Buoyancy class along with Underwater Photography. If you already have buoyancy under control then my next recommendation is Underwater Naturalist, which introduces you to the underwater animals you will encounter and how they generally will react amongst themselves as well as how they are likely to react to you as a diver.