It's a Monty Python and the Holy Grail spoof at the witch hunt ordeals by water: if an accused sank, he/she was considered innocent. If he/she floated, it indicated witchcraft. In this case, people try to come up with ideas to tell whether the accused character's a witch, and one of them's about throwing her to a pond and see whether she floats. The conversation has it so that the accused witch will be weighted against a duck.