First thing, before buying a housed DSLR system:
1)Manufacturers Catalog
This gives you a good idea of how can you improve the system and if there will be compatibility when you change camera/housing body...
2)Camera Owners Manual
Read it every day, they are very important!!! Especially on digital world... think this way: The old chemical lab was divided: one part is now on your personal computer but a very important part of it sits right inside the housing on your camera!!! Study it.
3) Your UWPhoto Instructor's Handbook and notes
Yes, best way to learn this art/sport/hobby is by taking real-world classes and dives with a good instructor.
4)Master Guide for Underwater Digital Photography by Jack and Sue Drafahl
It is a good book with lots of explanations but lacks on phisical concepts of some wrongly drawn images...
5)As many as coffee table UW Photo book you can aquire!!!
Dive, dive, dive... Shoot, shoot, shoot... Get informed on what other good UWPhotographers are doing/have done:
David Doubilet, Stephen Frink, Chris Newbert, Michael Aw... and much more.
This is good way to give the last touch you need on your UWP:
Develop your own language.
In this wonderful journey, if you get interested in General Photo, I recommend Galen Rowell's Inner Game of Outdoor Photography. This book may help you expand the limits of your art...very good.
Nikon School of Photography is also a good chance of improving you photography and broaden your vision.
Best