There are few scenarios where the G9's battery is not going to last you at least an entire dive, and in most cases, it will last you a minimum of two or more dives, even with the camera turned on the entire time.
However, with that in mind, I don't ever want to be in a scenario where my camera is turned off and can't turn it on in time to get the shot, or I've used up the battery when that once in a lifetime critter swims by me and I'm so low on battery power that I can't take as many shots as I need to!:depressed:
I'm not sure about you guys, but when I get a rare or special subject that's cooperating, I like to have my camera ready to go and usually I fire many different shots with many different setting, in hopes of getting one great shot! As an example, when I encountered this particular cooperative Lionfish in the Bahamas, I took a lot of shots, probably around 75, with lots of different settings, until I got the results I was seeking!
Consequently, I have several batteries for my G9, most of which are high powered aftermarket brands, and I keep myself in the habbit of always switching batteries as often as possible between dives, assuming of course that the conditions on the boat allow for it, that way I always have enough juice in the battery to keep in turned on and ready to go, so I never miss that one-in-a-million shot:!
