Excuse the late response to this but given I went through a similar experience a year or so ago I may have some advice too in addition to what's been said.
Manual controls are important...which camera has easy access to the menu/controls etc (menu logic isn't good for Sony from my on RX100 so makes things difficult)
Casing changes by model so realise you're locked with that hardware until you sell/replace. A newer camera version may not work with your original case.
$$ - don't scrimp and do your research. Look at the reviews as there is a lot of info on the web
Go-pros aren't the way to go...you have zero control!
Other accessories like slates, straps etc - budget for them too..you don't want to lose hard-earned kit!
Other observations (slightly OT)
How good are you with diving...you're losing potential use of both hands at times so think about not only your buoyancy skills but how confident you can do things without your hands. Also, adjusting white-balance means both hands being occupied if you use a slate/greycard. I really didn't realise the impact of this when I bought my stuff and only with experience will you learn but it is important to remember how to do things even with +100 dives. For me, losing the hands meant not taking the camera kit on night dives.
Last point - do you really care about the marine life? I was on a liveaboard with my stuff and whilst there were some excellent photographers, their behaviour underwater got me slightly depressed about the u/w photographers. These are experience divers but they way they set themselves up on subjects is shocking and the longer term damage will take years to recover. Multiply this by the number of photographers and you get a unrosy picture emerging. Those lovely macro pics you see are very intrusive to surrounding fauna...remember what you learnt on your open water about no touching. Try telling that to a u/w photog! If you do it, do it responsibly and with minimal impact to the surroundings.