Most of the information regarding box placement suggests that placement on poles keeps predators from reaching the nest. Also, the boxes on poles makes it easier for maintenance, observation. As for putting the box over water- I don't think that's essential. The box on a pole needs to be in the correct habitat, near or very close to water, not neccessarily on it. My boxes are right on the waterline on the bank. The ducks still like it and it's easier to get to.
