Choose a fluid ?
Pneumatics could easily run a motor - visit any garage and ask for a new tyre - pneumatic torque spanners, drills, etc are common.
You can only store so much air though, and when the "bouyancy tanks" are empty :?:
Hydraulic systems are commonly used where precision of position is required as there is much less 'spring' in the actuator.
I can't see any pollution problem with a water-filled hydraulic system, you also have the advantage of the robot being a constant weight (even if it leaks!) and so much easier to control its bouyancy, though the only bouyancy problem is likely to be staying down on the sea bed :!: