Hi Redboat.
You didn't say whether you were using a vented tank or a sealed ballast tank. I use the sealed ballast tank in all my boats and a centrifugal pump is used in conjuction. The pump will force water in under pressure (20psi), the air in the unvented tank is compressed by the incomming water and when the air reaches the 20psi figure, the pump will cease to deliver any more water even though it is still rotating.
This does not impose any additional load on the pump and won't damage it, in fact the pump speed will actually increase because it is no longer doing any useful work.
Stopping the pump will result in the water being discharged back down the tank feed line because the trapped air in the tank is pushing it back out and through through the pump inlet, so reversing the pump is pointless as it does not need this function to empty the tank.
A servo operated valve can be placed in the feed line from the pump to the tank along with a microswitch to turn the pump on and off. The valve and switch combination is arranged so the pump can be stopped, allowing the tank to retain it's pressurised ballast water, thus holding the sub down. Surfacing is achieved by opening the valve and allowing the tank to drain as previously described.
The sealed ballast tank requires to be built in a robust fashion and should be well engineered as it has to withstand 20psi max press.
Hope this is of some use to you.
Chris