Why seawater ? Just pump the released water back up. If the issue is contamination with brackish water from the lower part, I'm sure that could be handled by so

Why seawater ? Just pump the released water back up. If the issue is contamination with brackish water from the lower part, I'm sure that could be handled by some clever valve management - eq. release water to a freshwater pond when lowering a ship, so you loose only the water in the lowered chamber & can pump the water in the pond back up. Given the ships going up might import some of the brackish water when rising up, I don't think contamination can be fully prevented anyway. Sure, there will be power demand for all the pumping, but given how important this is I would think it would be economicaly sustainable- possibly with similar technology Japan uses for floodwater pumping in the GCANS project (gas turbine driven pumps).