A buoyancy-based lifting system can be dynamically controlled to lift, move, or hold a load an object of arbitrary size at a fixed depth. The control system can control the trajectory of a flexible gas-filled lift bag and attached load by opening or closing inflation and deflation valves based on ambient pressure readings obtained during a lift. Stable control can be achieved and maintained without having to directly measure the actual volume of gas in the lift bag, and without having to accumulate measurements of gas flows into and out of the bag. A controller can pre-determine limits of instability and lift control parameters, then use this information in conjunction with ambient pressure readings to prevent the system from becoming unstable during a lift. User intervention is generally unnecessary to maintain the system's instability.