I am unconvinced that manual white balance is worth the extra trouble and risk. I am a relatively inexperienced shooter underwater but have shoot quite a bit in drier climes.
White balance on land is great for correcting color temperature. The same thing doesn't quite exist underwater. A wide shot through even very clear water will give you different whites than a close up of sand. I am happy to shoot using auto white balance with an orange filter and then in post, color correct to suit my eye.
The other reason that I won't white balance manually underwater is that there is the risk that I will screw it up. I screwed up a manual white balance on one shoot on land and had to spend a lot of time fixing the color. I've only done it once, so I don't think I'll do it again, but underwater where I am working on my bouyancy and can't see nearly as well as I can on land, I am nervous about screwing up otherwise good footage by messing up a manual white balance setting.
I know some folks swear by manual WB underwater and I think that whaytever works for them is great. Personally I will stick with auto WB, at least for now.