Just remember that ISO is another trade off in photography. The higher ISO you use, the more noise you will have. Noise is especially noticeable in blues and shadows, so for underwater work it's something you need to keep a good eye on
The vast majority of compact cameras have noticeable noise at 200, fairly nasty noise at 400 and really disgusting noise at 800. Of course, everyone's tolerance for noise is different. There are also plenty of noise reduction software choices available, but often they hurt an image more than help it.
The rule of thumb for all cameras is to use as low of an ISO as you can to minimize noise. Of course, this will mean you have to trade off with a slower shutter, a wider aperture and/or more light on the scene with external lighting.
Getting close to your subject so your internal flash becomes your main source of light allows you to shoot at a very low ISO and is one of the strengths of any small camera system underwater.
As it's digital, we have the luxury of experimenting even while we're underwater so run through the various ISO settings while you concentrate on one scene. It only takes a minute or two and will go a long way to showing you a baseline for what produces images that suit you. I'd do this with three or four quite different shots to really get a feel for things - a nice macro, a portrait shot, a full body portrait and a wide angle scene, for instance. I find doing this little "test" is also helpful in establishing particular base settings for other options on your camera, too.
