Just like films in the old days, batteries cannot be changed underwater (it would be neat but probably not really environment friendly). So when you chose a camera it's important to select the right battery as well. Battery life is a major factor as it defines how many dives you will do on one load of the camera. It's very significant when you're diving in hot and moist climate where camera fogging is your dreadest enemy (see our tutorials for details). Even in temperate climate, opening the case may cause humidity to penetrate it. While underwater the heat generated by the camera (especially in a small enclosure with a lot of flash firings) may cause that humidity to generate fog. And you'll be sorry. So if you want to be able to dive your four PADI regulated dives a day without opening your case, opt for a long battery life camera. As a rule of thumb if you're crazy like me, you'll take 100-150 pix per dives, 75% with the flash. Cameras with 4 AA batteries seem to exhibit a longer life than those with 2 or with a single Lithium battery. 4 AA also make the camera heavier which gives a better grip underwater and makes the camera gear slightly buoyant instead of shoot-to-the-surface-in-5-seconds buoyant.
Another factor is whether or not your camera will accept normal AA batteries. Although most decent dive boats offer electricity on board you may forget to charge your battery due to the amount of Tequila you drank last night or just because you found it more fun fooling around with sexy locals than plugging all your stuff before you went to bed. So anyway, if your camera works with AA batteries and you brought a few with you, you can easily swap them out. You don't have to wait hours for your NiMh battery to recharge. You could also have brought several pre-charged batteries but be aware that most of them don't hold their charge very long (a few days at most).