An otter is an aquatic predator and ducks are an aquatic prey species, that is all I know except that an otter will kill and eat a muskrat if given half the chance. If the gators return to your lake the otters may find that they have important business elsewhere, but so may your ducks. I know that isn't much help, or the kind of answer that you wanted, but everyone else has already given you very good advise.