When I was diving in Belize last month, I set the white balance on my first dive when I descended to the bottom which was at 45 feet. I used that setting for all my dives from 10-100 feet and it worked great. I think once you get past 30 feet or so, the color difference is gradual between 30 and 90 feet and a simple tweak in photoshop can correct any minor issues. The s50 has 2 user white balance settings that it will remember for you for future use.
To answer the previous question, I don't use any external strobes, just the in camera strobe, and only up close, like into a hole, etc. The backscatter is really bad. It works great for shooting lobster or fish hiding under a ledge, but horrible in caverns and swim throughs.