There are plenty of places to learn about underwater photography, and fortunately, many of them are free (not a term you hear much when discussing diving or photography).

Check out these resources:

• Digital Diver
  
• Includes tutorials, an active forum for photographers, classifieds, a trip finder, a Depth of Field Calculator and a Strobe Finder

• Wetpixel
• Another great forum, product reviews, galleries, and more