Here is what I would do ,use hydraulics to move the piston in the tank or tanks,it cheap easy to make and very very powerfull,infanatly adjustable and power as well, use any type of ram but syringes are far cheaper and never corrode and can be cut to any lenght.

Hydraulics controling a piston ballast system thats the way to go very simple no gear drives to strip out or switches to fail or motors to get hot as even the pump can go in the wet!

The same system I use for the masts see the video lifting a battery thats a small syringe imagine a large syringe! You will be astonished just how powerfull that is if you set it up the wayI do.
Also you can directly actuate those pistons you making by just pumping in the water and out which is themost simplest form,and as many as as you like can be done each on a seperate channel or all at once.


Dave amur sub Yard
http://www.youtube.com/watch?v=rYy_ivqFO8I