Using Q and Z makes sense for buoyancy control when diving as this translates to ascend/descend controls for helicopters.
Having a default binding of X for ascend makes no sense, it took me a month to realise that there was even an ascend function!
As a secondary (less important) issue - the descent animation is not correct. It looks like the diver jerks around to get down, and on ascent the diver does a retarded frog kick which is also inaccurate. Divers control their buoyancy through breathing techniques and BCDs (Buoyancy Control Device) - which doesn't really have or need a visual representation (especially for a rebreather setup where breathing doesn't really affect buoyancy).