I have used air motors several years ago on a working ROV.
Here are some recommendations.
Use a two way air bank, I recommend Bimba Brand.
You will have two use many through hull fittings so the air bank
can ride inside the housing.
Port the exhaust out into the water. send air down at 160 PSI.
Regulate the flow control on the exhaust side rather than intake, this allows for
greater control. of motors and actuators.
Make sure your air bank has two open close ports for each actuator you will
be using. in other words one port will let you run one motor on/off, while
you will need two ports for each piston or actuator to move an arm.
If you are building a small enough robot you can use Die Grinder motors
they are cheep and usually made of high grade aluminum.
They also have a nice chuck design to hold your home made prop.
another nice thing about air is you can use bladders to bring your rov]
up or down. In other words a strong claw and your bladder could lift heavy
objects to the surface that most motors could not.
---- Here are the down sides to air. -----
No revers, most air motors only run in one direction, I solved this by adding extra motorsmounted in revers direction.
You will not be able two push depths greater than 200 feet. The only way around this
is to go two high pressure air such as 3000 psi dive bottle or dive compressor. Once
you step up to high pressure air bank valves the cost goes through the roof.
At that cost you may as well go hydraulic, more power, reliability ect.