Can the built in flash be used underwater? How's the lighting with that? I assume the backscatter is a PITA...
On some systems (ie, "bags" ... which I'd never, ever trust with an SLR), the answer's probably yes.
However, the general problem with backscatter is that it is a function of strobe-subject-lens geometry.
The general technique for avoiding backscatter is:
(a) don't kick up crud
(b) position the strobe away from the lens
The reason why (a) works is pretty obvious, but (b) is not quite as obvious.
The simple reason why is because when it is angling in from the side, the amount of water that is bathed in the light of the strobe *and* within the field of view of the lens is smaller than if the strobe were right next to the lens.
To illustrate this aspect, visualize the following:
#1: You're in a car (stopped) on a road with those reflectors in the lanes. With your headlights on, you see a row of the reflectors down the road in front of you...a whole bunch of them, all the way from your car to that stop sign way down by the street corner. The reflectors represent "backscatter" from junk in the water and the stop sign is your subject.
#2: You're in your car again, but this time with your headlights off. Instead, there's a cop car a short distance down the side street and he has his floodlight turned on and is illuminating the same stop sign. You can see the stop sign just fine (same amount of light on the sign), but because his light is coming in from way over on the side, none of the reflectors on your road are lit up, so you get a cleaner view of the stop sign.
The difference between #1 and #2 is the angle at which the light source gets to the subject (stop sign). From there, the light reflecting off the sign comes back to you (car = camera lens) and is travelling the same path. By moving your light source off of your highway towards your subject, the reflectors along your sightline didn't get light, so they didn't pollute.
That's the basics. To get a little more advanced, think about the phases of the Moon. Essentially, how much of the Moon is bright/dark depends on the angles, which when we apply that here means that the angle that our strobe has to all of those particles is like illuminating a hundred tiny "moons" in the water: if the strobe is right next to the lens, then they are are lit up as if they were Full Moons, but if we move the strobe out, then we get Half Moons ... which makes them less distracting.
Putting these together, the reason why moving the strobe out to the side minimizes backscatter is that it illuminates "fewer moons" and the moons that it does illuminate aren't Full, but only "Half" or thereabouts.
Thus concludes today's combined Geometry, Optics, and Astronomy lesson. I shoulda just found an illustration on the web
-hh