Note the white line in the picture. I've encountered this at least three times. There are no obstructions, yet the dupes will not path past a certain point in the water. They were previously able to walk all the way to the edge (they built pipes over there), and seemingly nothing about the area has changed since then.
Manually moving a dupe to that area will fix the pathing issue, but only for that specific dupe.
Usually happens with a long body of water.