How much to pump the water back (either fresh water from the bottom lock or replace the whole flight with salt)?
200,000 tons of water per ship, 26 metres above sea level, about 200MWh, so call it 500MWh. With 20,000 ships a year that's 50 a day, or 25GWh. A 2GW solar plant
A large solar farm would can reach that capacity (you don't need to run the pumps 24/7, so Solar is perfect)
The canal also supplies drinking water for Panama's second largest city (Colon), as well as several smaller municipalities. On top of that, replacing the freshwater would kill many of the plants and animals living in and around Gatun Lake. It would be a disaster.
An artificial lake created in part by damming a river system.
If you don't think that would have much impact, start advocating to alleviate western US water concerns by pumping from the Great Lakes and reversing the course of the St Lawrence River to flow from the Atlantic into the lakes. I'm sure people will be very receptive.
An artificial lake created over a hundred years ago now, with it's own ecosystem that has developed around it.
And rivers still flow from that downstream, likely entering the water table of a much larger area than the lake itself. Despite it being "artificial", the water still passes through it to downstream ecosystems, as it did before any dam construction. While they may be lower volume due to the drought and limited releases from the lake, replacing that with "Salt water or nothing" would still be a massive change.
