Unless the tank is vented e.g. not pressurized, the boat won't hold it's trim, as gear pumps leak and air pressure will push the water out. You'll be going up and down like a lavatory seat. If you want a simple solution get a peristaltic, that will work perfectly, but I'd recommend you also fit some sort of mechanical failsafe to avoid over pressure. The TT Neptune has pressure switch inline, which shuts the controller down once a certain pressure is reached. You can also get these from Norbert Bruggen as he uses them in his Delta sub kit.
Either that or make a small pinch valve. I passed details of small pinch valve I made for the Holland on to Kevin, it uses a small 9 gram servo, so it's small (didn't have room for normal size servo). Nigel had fitted a clippard valve originally which worked, but they're designed for gas not water, and the iron core is exposed to the water, so it was corroding.
The other option is a festo ball valve. I didn't have room for one of these, but they work very well. You have to modify the servo for 180 degree rotation and make a custom arm for the servo.