I'll disagree with these guys.
The cost of the camera is only part of it. The cost of a housing and ports and so on, is considerably higher, for slrs. Also, the weight, and bulk, are much higher, so if you travel for diving, this is significant.
For me, shooting with a nice big bright LCD display, that is fast enough update, and low enough lag, is superior to using add on optics, to try to see through an SLR viewfinder.
Underwater, any lens beyond about 100mm (35mm equiv), is not any use. The water just takes away the contast, too much, beyond 6 feet or so.
So, the 8800 is not any use. The 8400 is very interesting, due to the wide angle, but reports are the low light focus is quite bad, and the LCD freezes briefly, during the actual auto focus operation. For me, this is a killer.
The V3 has an autofocus system that ROCKS. Probably as good as the digi rebel. And it has a world beating 13 millisecond final shutter lag. (Which may bet longer when using flash.). Only drawback of the V3 is the non-buffered RAW mode, so the cycle times in RAW are about 12 seconds.
The canon Pro-1 might also be a consideration. Price way down, and they released a firmware update, which makes the autofocus really good, and improved the electronic zoom response. Good raw mode cycle times.
HTH,
Don