The modern cameras do a lot of things by themselves and so no one needs to deal with light, apertures and depth of field anymore. And that's a good thing, because the UW filmmaker has enough to do with his environment.
The most important thing is a perfect buoyancy and by that I mean really perfect. The diver must be able to maintain his position for a long time to the centimetre. This is no longer done via the buoyancy compensating vest (you won't have a free hand anyway), but exclusively through the lungs. The film is filmed from practically all layers, upright in the water, lying above the sand (not in the sand) or upside down, if there is no other way. But also on the back diving against the water surface, extraordinary pictures are possible. Note: the more exotic the angle, the more interesting the shot becomes.
In addition, the filmmaker must of course be able to control his camera blindly and know which button does what. If you press something wrong, you lose time and most of the time you lose the film motif. The statement sounds banal and it is, but imagine a situation where suddenly a whale shark emerges from the blue water. Then every grip must fit.