Hi pledosophy, I use a gopro black almost daily diving.
Here's a few things off the top of my head that may help.
Get familiar with using the buttons with the case ON, in your house before you go diving. Once underwater it's a lot harder to shuffle through the right modes.
Your video will be much more jerky than you think, so go slower than you feel is natural and pan slowly.
I use the headstrap while diving and for me that is the best. but I have almost lost it a few times from swimming to fast and forgetting its on.
I run all my videos through adobe premier, there's is a tool called warpdrive that will smooth out any video. It's pretty amazing, I could be in a 10.0 earthquake and as soon as I apply this filter in post production bam, it's smooth as butter.
The gopro batteries are really crappy. They last me 40-50 mins only. I'd buy 2 spares before you go.
If you are prone to dropping things gopro makes a floating case that you can attach so if you drop it it wont sink down to Davey Jones locker.
Use at minimum a 32gb microSD Card, I like sandisks extreme
last tip is remember that if your topside you wont get much audio with the case on. wide mode is best, super wide gives too much fisheye
Have fun on your trip!
Tom