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!
|IMG_8168ABCD by citori525|
|McKinley meadow by TimR32225|
from Natural meadows
|Flare-well to a Classic Flying Machine by cjf2|
from Flying Machines
|_DSC2146 by jerste|
from Helios-44 II
|Leopoldsteinersee by RaCor|
from Landscape - Colour #3