Is the issue just that the fill rate from fresh sources is too slow to handle the previous volume of traffic or that it diverts fresh water away from downstream consumers?
I think just the former. Considering that the locks are basically adjacent to the coast on each side, there’s not much “downstream” to speak of.
(Also, agriculture doesn’t appear to be much of a thing in the immediate vicinity of the canal anyway. The surroundings are basically either all protected forest or urban area. I guess farming wasn’t allowed when the Canal Zone was US-controlled, and the Panamanians just sort of kept that policy afterwards.)