Might want to do some research on where the the water in the canal comes from and why there is a canal. Hint- it's not salt water!
While the Panama Canal does get the bulk of it's water from fres water runoff, it also has some major lakes involved. Besides that the canal is
open to the ocean at both ends.
That means that except that the central area of the canal is at a higher level, salt water would invade the canal much farther inland.
THe same as the Corinthian Canal..open both ends and water tends to find it's own level. You can even do an experiment at a minute level.
Next time it rains, find two puddles in your lawn or driveway and dig a trench between them. Does one puddle still maintain a higher water level
than the other, or does it find it's level between the two?
BTW: Pelopennesia used to be an isthmus, until the small connecting bit of land was excavated..