Hi
There's no need to put the post holding the box actually IN the pond...assuming you have somewhere on land to mount it...the one caveat being that boxes on land need to be higher than boxes over water (at least 10 ft. in most cases) and absolutely need a predator guard intstalled around the base of the post/tree to thwart coons and sundry other critters that will be tempted to investigate a potential meal. A sheet of lightweight metal, of the type used for autorepair, will work just fine, assuming the tree or post has no connection with other trees above ground (interlocking canopy, etc.) The metal prevents critters from getting a grip on the post.
I've maintained up to 50 wood duck boxes over the last 20 years...I've had woodies nest as far as a quarter mile from the pond, and I've seen them in natural cavities 60 feet up in an old silver maple...that particular hole has produced broods of ducklings every year since I was a kid.
Good luck and enjoy your pond.