I'm guessing you're not using the strobe on every shot, and do a lot of natural light work? And what's your LCD timeout set to? And how many shots do you take per dive? Or are your dives just shorter than mine, usually?
I consider a typical dive for me about 45 minutes, I am often deep as well. I run my internal strobe on Manual and low power and I shoot about 80% with flash, give or take.
Diving the cold, dark SoCal waters, I shoot 100% of my shots with an external strobe (full manual mode, internal flash set to minimum output). I find I get roughly 1.8 dives out of a fully-charged Canon OEM battery. (I think my Wasabi batteries are about the same.) Obviously this depends on dive duration, too. Figure one hour dives, on average. And I'm often shooting 100 photos per dive.
I was able to make three trips this year before breaking my leg, on all of them I got at least two solid dives and usually about 2.8 dives. I take about 60 to 80 shots per dive. So, 2.8 times 60 is, what, 160 shots? Well within the advertised specs for the camera, I have gotten as many as 200 shots, 2.8 times 80 is 224 so again within the advertised battery life.
A curious thing, beats me, my #2 camera runs longer than my number 1 camera, about 20 shots.
This means that, the majority of the time, my camera dies in the last few minutes of the second dive, IF I don't change the battery. As a result, I change (usually... and pay the price when I don't!) batteries after every dive. Very annoying, but I can live with it, considering it's among the S90's very few flaws.
I consider the S90 to have sufficient battery life for two dives, however, it is at the low side of what I would want. Since the S95 shows less shots per charge, not sure about that, glad I have the S90, I like it better than the S95.
By changing the batteries on every dive, I can be sure my camera lasts the whole dive, every time, even on my nice long (occasional) 90-minute dives.
Good insurance given your long, shallow, flash intensive work. I usually change out between dives two and three if I am expecting to need the camera to the very end of dive three.
As to my settings, I have the screen set to 2 minutes I think, my IS set to shoot only, all of the safety stuff and red eye and focus assist light are set to OFF and I have the camera Auto Off turned OFF. Once the LCD screen goes dark I need only shake the camera or touch a button to wake it up quickly, I also have the lens retract OFF. I also set my screen brightness down below default as it is plenty bright underwater at the reduced level. I never use the zoom, maybe once in a blue moon, so that saves battery also. I also often use manual focus and just zone focus for my expected shot, that saves battery power also.
I have not been cleared to dive yet, I have a non-union in the femur bone, I cannot carry the weight of a tank without pain, probably additional surgery required before I can dive or run again though last night I swam 2.5 miles. So, I may just sell the stuff so it does not go to waste and start over when I get all healed up, maybe, maybe not.
James