So instead of keeping the water in a tub, you might need to keep it in the pond so that the ducks will hang out there. If their water is kept in a tub, they won't do their sealing work on the pond. Feeding them lots of greens where you want them to work will probably help as well.
You are absolutely right. The water goes on the bottom of the pond, the ducks go in the water.
No matter where you feed them, it's in their nature to "filter" the mud on the edge of the water with their beaks.
And those banks you've created without reinforcement? they will be gone. The ducks will level them out..
I read in a book that more than 6 ducks per acre of water will destroy the perimeter because of this.