If I wanted to learn photography I'd go with a photographer rather than an instructor. I believe a course or even seminar is going to get you more than a sanctioned UW class or another plastic card.
For trial and error study go with a digicam. Even though the inital expense is larger, it pays off soon as you save on processing & film. A good 3mp camera will do to get high quality prints if you know how to use it.
Anybody really serious about UW photography can contact Brooks Institute of Photography in Santa Barbara, CA. Very good school that has a 2 course underwater programm (each 7 weeks long). During breaks they usually have underwater workshops, and the Channel Island location ain't bad either.
http://www.brooks.edu/degrees_ss.asp
http://www.brooks.edu/underwater.asp