is more lack of colors rather than light. In most occasions there is enough light down there even to use 50ASA. Of course, it depends on weather, hour of the day, waves, clouds, visibilty etc. And your lens, of course. If you have a "slow" lens then you may need "faster" films.
However, even in the best crystaline water where you have plenty of light you lose colors as you go deeper.
Although in the last few years colour film technology has improved a lot and there's plenty of good fast film even 800ASA and 1600ASA, I still think that a 100ASA film gives better colors and sharpness than any "fast" film I know of. So I'd go for the slowest possible film. However, if your lens widest aperture is, say, f/5.6 then you may chose 200 or 400 ASA. Remember that most of the time UW you'd need to be able to shoot at speeds in the range of 1/30-1/250 second, aperture limited to whatever your lens is, usually you'll shoot at f/8-f/22 in some cases you'd like to use the widest aperture you can afford, And you'll take photos of subjects at about 10cm to 2meters away from you.
BTW, in case you consider using a strobe (or for others who read this thread
) then perhaps you should choose film speed according to your strobe's guide number, lens aperture and distances that you are going to shoot. For example: if you have a strobe with a GN of 20, then using f/11 and 100ASA film you can shoot with your strobe to illuminate a distance of approx 2 meters (GN/f# or 20/11) which is more than enough for underwater photography. If you'll shoot closer then you can use even f/22 from 1 meter and so on. However, you also need to be able to "balance" the strobe's exposure with the background. At night there's no problem because the strobe is anyway the only source of light. During daytime you can take a slightly faster film so you can take pictures at relatively high speeds like 1/60 seconds or even faster (if your camera/strobe can support it).
So, I hope I didn't confuse you, just wanted to say there's more to it than just thinking there's not enough light in the deep
EErrrrr.. and without a strobe you can forget of getting colors below 3-4 meters deep.
