You don't say what your underwater photography experience is. If you have little or none I strongly suggest you start with a reasonably good digital compact in the manufacturer's waterproof case. Quite a good idea to add a single external flash/strobe, but try diving both with and without it. And to take photographs underwater, you need very good positional and buoyancy control - you need to be a good diver.
Looking again at your post, most of what you describe as "the technical side of underwater photography" is also the "technical side" of any photography. You need to have a good grasp of what histograms mean and how to use them, what focal length and depth of field are, and manual setting of cameras before you ever venture underwater. And of course how lighting works and the effect of different lighting on a subject. All of this you can learn either by teaching yourself from your camera's own manual, or from any of the good books and internet forums there are. A good start for the latter is http://photography-on-the.net/forum/ . Then you'll be ready for the add-ons that being underwater will add.
My Friend,
You are the one who helped me figure out which SLR camera to purchase in the first place!
AND I thank you for that, your help has been so valuable!
I used a compact digital for years & feel I mastered that with the best possible results, however I wanted (& got) an slr camera to try to achieve greater detail & better overall quality photos. I have already gotten better clarity in a
few pictures by chance with the SLR on my first & most recent dive vacation. Playing around with it was fun, but also frustrating, because I didn't really know what I was doing & therefore overall did not get the kind of results I wanted.
A great point you make & the most logical is to master the camera out of the water- duh- why didn't I think of that???
I tend to leave the SLR on automatic above water which happens to gets lovely results, though that doesn't seem to work for my photos underwater. I will most certainly heed your advice & I will work with manual settings on land & try to master (or at least get a feel for) the settings. I will check out the link too.
Thank you!