If I may share my very short experience here:
1. for the backpack you have to use the battery backpack if you want to film during more than one dive. Changing the battery on board is always possible, but opening the camera case in a wet boat is always tricky and you need to store the batteries, etc... it just add complexity where you search simplicity, this why we bought a gopro and not a sophisticated camera. So you cannot use the LCD one that in any case drains a lot the battery.
2. handheld the camera with just a safety lockline, is a cheap and not so bad option. It gives you a lot of freedom and I'm quite happy with the footage I have done with (
Hannan1 2 - YouTube ).
May be I will change my mind in the future, but for the time being it's good for me above 30m and outside wreck where of course you will need lights and so far tray.