I would highly recommend starting with the TG6. ITs a great camera to start with, its small and compact. If your are staying shallow then you dont need a housing. if you go beyond 50ft then you will need the housing.
Check out WetPixel.com for learning about underwater photography.
Check out Underwater Photography Masterclass (link) for a good book on the subject.
check out Backscatter.com to learn more about the TG6 and how to set it up.