The WP-DC6 case does not appear to support an external flash. You would want the flash in a case of it's own.
Using a light source would definitely help, but bear in mind that it doesn't have to be a flash. If you can find a decent underwater flashlight that you could hold, or mount to a helmet while operating the camera, a lot of near field photography will be improved. Using an underwater flash was about the only time I accidentally discharged a high-voltage capacitor, a mistake I hope to never repeat. Back then it was all film based and the camera had a wire frame in front of it's lens to save you from having to look through the view finder with a mask on. I suppose LCDs on digital cameras are another solution to this problem.
A recommendation I do recall from a friend was to use a camera with RAW support so that the colors and exposures could be better adjusted later. You'd think a specifically waterproof camera would be worth the purchase price of a fraction of the cost of a ticket to the Maldives. I've personally tried an Olympus Stylus 850SW, but the 3 meter water-proof models aren't really that practical (though they do have a mode that does a good job on color compensation) while the 10 meter water-proof models probably are best used for snorkeling and probably for less than a couple hours. My 850SW eventually sprung a leak after a 20 minute session, one of only 3 underwater sessions. Customer service did repair it, and the SD card and photos were fine. Also worth noting, all these products (the cases included) have a replacement schedule for their gaskets. The 850 SW is supposed to have them replaced annually, and I can't guess as to the cost, but it's likely non-trivial.
