H-Bridge & Actuator (for ballast tank control in AUV)

However you have raised a new problem for me. With no internal feedback available, could i simply activate it and record the time taken to travel a certain distance then incorporate that into my code?
Might work esp if the load is a fixed amount and your accuracy requirements is not too high. It's certainly worth playing with.

Or would that be prone to errors.
There is always some amount of error no matter what method is used, it's a matter of determining what accuracy you actually require for the project and then seeing if you can obtain it using timed movements.

If not is there some sort of external system that could monitor position?
Well there are linear pots that could be mechanically coupled to the actuator although that is not always easy getting the mechanics right.