The second thing I was told to look out for was back lighting. I was told to always try to use natural light (and open water when you can) as a back drop. This gives you a better outline of the subject, gives better color definition to the plants and sealife behind your subject, and helps create a solid "keeper" for the photobook. To do this always try to shoot at an upward angle. Thats not always possible, but it helps a bunch. I once saw a sea anemone in a conche shell at about 25 feet on a rock jetty. Visibility was about 4-6 feet and the water was green. Not ideal photo conditions. I set up a little lower than the subject, got as close as I could (I turned the MACRO function on) and took about 5-6 shots. One of the first ones came out nearly perect. The sun popped out and shone on the shell, making it transluscent and providing a good background definition and made the shot a keeper. The following shots were taken as I ascended a little and the color/lighting changed drastically. I didnt have a strobe, just a standard camera flash that I diffused. If I would have had a strobe it would have been a magazine quality shot. If I can find the shot I will post it here.
Like I said, Im not a professional and my advice may not be perfect, but it has worked for me. good luck!!
[http://i303.photobucket.com/albums/nn126/debay777/PICT0044.jpg