Not a diver myself, but did work on some friends pictures. From our discussions:
1) useability of strobe depends on clarity of water. If lots of particles in suspension, not much use (like shooting strobe in fog).
2) try to keep distance to subject to a minimum: a "thick" layer of water will alter quality (sharpness + color). Again, all dependant of how much particles in suspension (seen fantastic - professional - movie of diving under the polar ice cap with excellent clarity)
3) depending on how deep you'll be when you take the picture, the color cast on the pictures can become important. Water absorbs red radiations more than blue ones, so you'll generally end up with a blue color cast. Can be (at least partially) corrected in posttreatment. Strobe can help to lower the color cast.
An example of correcting color cast in post:
Have a nice trip!
|Christine by JP Zanotti|
from Car wreck
|Fangorn Forest by cand1d|
|Yosemite Falls with Moonbow by Jonathan Shapiro|
from Best Landscape of the Week 4