For full-on flexibility you'll find it hard to do better than the Olympus c5050.
Take a look at
www.uwpmag.com and download issue 14 where you can find a review of the camera in an Olympus housing. Then also goto
www.wetpixel.com and
www.digitaldiver.net and browse through the digicam sections of these for more info and for photos / links to others people websites for example of what the 5050 can do. Keep an eye out also on the classified sections of the sites as complete systems often come up at good prices. ebay is also worth checking out regularly - one complete system in an Ikelite housing with Ike strobe recently went for about £700 which was good value. Don't pay more than £250 for a second hand 5050 camera (and at that price should include extras also).
Also do a search on here for 5050 as loads of us own them and have posted stuff all over the place. If you go to
www.ingmire.net and look at the red sea stuff you can see some photos I took in '03 - I've some better ones from '04 but not posted them yet. These were taken without an external strobe.