Swimming would be O.K., I do in mine. But having the ducks in there is just asking for problems. My neighbors pond was muddy to a point too. Once the pair of ducks were removed, the pond cleaned up. I would have never guessed that only 2 ducks on a 1/4 ac pond could muddy the water, but that's the only explination.
That planimeter is more accurate the larger you can make your pond (zoom in).