for submarine ballast, forget flooding tanks!
These days model subs use a bladder, just like fish do.
It's either on a lead screw or uses compressed air to force water in or out.
The other way to do it is make you sub' have neutral buoyancy, then use dive planes and forward speed to dive.
Or, again neutral buoyancy and use small thrusters to submerge.
Some of the ROV designs these days a re pretty cool, using bilge pumps for thrusters.