So far I've done several tests with the gopro, and the battery life with just the camera running is good for what I need. I ran two tests today (with the lcd screen automatically switching off after 1 minute), and the batteries ran out at the 1hr 45min mark. One of the tests had the battery freshly charged, and the other had the battery charged over 12 hours ago. I'm _kinda_ convinced there's not over-excessive battery drain now. But I'll keep an eye on it. Probably the very first charge and its super miserable performance just didn't hold very well.
That said, I find it surprising that the lcd permanently switched on lasts about an hour and a half, a 15 minute difference from the lcd off.
A quick google indicates that for newbies, a tank of oxygen will last about an hour for the n00b depths I'll probably be diving at, so this bodes really well for what I need it for. In a good situation, I can turn on the recording before I dive, and it will probably still be running after the dive. So, if I am going for a few dives in that day, I probably need to bring a few batteries.
During these tests though, the camera runs really hot; the last test I ran, the button besides the lens was uncomfortably warm. Hopefully, the camera will run cooler underwater.
Can't wait to use it for timelapse and other stuff! Sooo exciting!