Thx guys. How about a complete fill, with no care or finese, then a complete extraction. With four dams keeping the water out, you can build on the sea floor in broad daylight for as long as you need to. The final act is removing the dam walls, let the water flow over the top and your structure is now under water.
You are filling/extracting the same volume (possibly) but have complete freedom of movement. My only concern is ensuring the water is 'smooth' after the dam walls come down.