William's advice is good - an external strobe is very important for good looking pictures.
However, if you are only working very shallow (<2 m), you might be able to get away with just white balancing later when shooting RAW, but the results will still lack the colour and quality you would get with an external strobe,
The camera has sufficient battery life to fill a 1GB card using the flash on all shots. I'm not sure how many photos you plan to take on a dive, but this gives you around 100 - which isn't too bad. As far as the issue of the lens crashing into the housing - I have only ever used the Fuji OEM housing with this camera and it worked fine. I did use the INON AD105 too which works very well with this camera giving a nice wide view.
Regarding William's concerns with synching with an external flash and backscatter - I wouldn't be too concerned about that, the INON clear photo system works very well at eliminating this problem with their D2000 flash (which is the one I would recommend for the F810).
There are a number of users working with this camera over on digitaldiver.net. Search for blaze, dr bob, jack connick, to name a few and I'm sure they will be able to provide you with further advice.
The AF system on the Olympus Cx0x0 series isn't nearly as nice as the Fuji F810...