Based on the diagram of the locks and water saving basins at https://en.wikipedia.org/wiki/Panama_Canal_expansion_project... they could have a second basin D at the same level as basin C. They would pump the fresh water in C that has come down the lock chain back up into the lake. They could pump sea water into D that would be used to fill the bottom lock when a ship is acsending the chain. Then the sea water in the bottom lock would be drained to the sea when a ship descends.