I've been using a single housed Nikon flash for the last six years, and I love it. First of all, the SB800 is a great flash for land photography. The flash works so well with a Nikon DSLR that you have to try it to understand how effective and how flexible it is.
The guide number underwater is comparable or higher than most underwater strobes, so that's not an issue. I have a Subal SB800 housing, which is expensive but well made. I have friends who use the much cheaper FantaSea SB800 housing, and although some of the early production units weren't very well made, I think they've gotten their act together now.
The Subal and FantaSea housings have all the flash controls, so adjustments are easy to do underwater. I shoot in iTTL, but it's very simple to adjust or bracket exposures with the flash controls - much easier than it is using the camera controls, and the large digital readouts on the SB800 are very easy to see. The SB800 gives very good results in iTTL, although I do bracket shots.
One very nice feature of the SB800 is the ability to automatically or manually zoom the beam width, which in effect lets you diffuse the light as much as you want and still have a decent guide number.
The biggest disadvantage of a housed SB800 is the size and weight, but to me the results outweigh that, and some of the bigger digital underwater strobes aren't that much smaller.
I have a Fantasea SB800 housing and a second SB800 that I may try firing using fiber optics linked to the first SB800, but I haven't tried it yet.