I have a D10 point and shoot, and about once in 100 shots the fish, my ability, and the slow autofocus allow me to get a shot I really like. I suspect that the newer D30 is better, but not enough. I have some big trips coming up, and would like to have faster autofocus, a better sensor, and external flash.
On the other hand, I do a lot of free diving and only a little scuba. A huge housing for my 5D III and six feet of flash arms would be too much to move around with. I'm guessing that a fairly small external flash and arm for snorkel shots to 20 feet (depth, not subject distance) would be enough, but with the option to change to bigger flashes for scuba.
I saw some good posts from the past, but the model information is probably out of date. What is out there these days?
Oh - my budget is somewhat flexible. To get most of what I want for under $1000 would be tempting, but I could go quite a bit higher for TTL-capable flash and a not-too-tiny sensor. The only thing non-negotiable is focus/shutter lag. I'm sick of waiting 5 seconds or more for the shot to take while my composition goes from good to bad to hopeless.