Think it will be overcast, 3 dives a day so anytime other than dusk or dark, could be anything up to 30m, and can't answer viz question. I guess ISO needs to be 200/300 and speed priority around 1/100th roughly.
Ok being serious for a moment, I'd suggest putting it in shutter priority at around 1/200th with an ISO of 800. The S95 has excellent low light performance so 800 shouldn't be too noisy. If it was me I'd be in full manual and adjusting on the fly depending on what the conditions were at the time and which direction I was facing. Remember if the camera is pointing up then it will be exposing for the surface rather than the sharks below it so you may need to over or under expose with compensation to allow for the surface light. Hope that's a bit more use than my 1st comments
You need to establish the highest ISO setting you're happy with (ie your personal limits for the amount of noise you'll accept).
Next you need to determine the style of shot you're going for - action shots require a short shutter speed for example. If you're trying to shoot moving sharks, then you need to have a shutter speed to match. This would be 1/100 or possibly 1/60 if you are pretty close and have a decent w/a lens. You can accept a slow shutter speed if you're lighting the subject mainly by flash as the flash time is 1/1000 second or less.
So using TV for 1/100 with 400 ISO (say), you may or may not get a decent exposure. If the shot is too dark, then up the ISO and live with a noisy shot, or get closer or wait until it is sunnier or you find sharks nearer the surface...
In short, there is no way anyone can predict the "best settings" for you. You have to take the conditions you have on the day into account - they have a H U G E impact on how to set up the camera. Sorry, but it's the same for all of us.