Amigos,
I'll wade into this pool
Any accessory strobe will help whether shooting with a Compact, Mid-sized or SLR system. But I've also seen people in bright tropical conditions get nice photos when shooting close and down INTO the reef background using just a built in flash on their compact camera.
Or also turning the flash off and using RAW and bright available light at higher ISOs. A WA lens on compact housings can do a great job in decent light.
IMHO I also LIKE the built in fun scene modes.......They're fun to play with and isn't that what shooting UW images is supposed to be about?
The final thing I'll say is people get crappy shots because they talk and talk and talk, lament about buying more gear (which is what started this thread) but never even go to a damn swimming pool before a trip and try their set up out........
Sigh................
One of my Blog posts on my web site related the "Run What you Brought" idea...........
www.haasimages.com and hit the Blog post link........
I tell folks I didn't get decent photos because I'm lucky, gifted, or whatever......
It's because I've put my time in preparing, practicing and then getting "OUT THERE"
Just one old geezer's 2 cents
David Haas