All of the above is valuable information & I'll add another important tip. Know your camera, and how to change settings quickly. When you add a housing that sometimes takes a little more thinking because of the button placement, so practice using the camera in it's housing. One other piece of advice I've learned relates to the need for cheap reading glasses for things close up, which I can't wear while diving, so my options have been to either buy a mask with optical correction like a bi focal lens (which I haven't done because I get by using memory) so if that's applicable learn the order of your menus so that even though you might not be able to read them properly you'll recognize what each page & line relates to. Good underwater photography still relies on several basic photographic rules, and about the most important one is knowing what makes a good exposure, and whether you should be using Av mode, Tv mode or full manual. A lot of people trust the P mode (or auto) but that's not getting the best use of your gear.