Well, you could always use the very tacky but effective way of anchoring things in a waveless body of water- fill up old (well-cleaned) clorox bottles with concrete, let them cure, and securely tie boating rope to the handles. Sink the bottles and float the box from the rope. Ugly but that's how I've seen it done with rowboats, etc. in shallow water. Cinder blocks work equally well. Sorry, not the most aesthetically pleasing option, but it works.

There are no trees around the pond?...I screwed my boxes to adjacent trees. For protection, I screwed aluminum flashing around the trees under the boxes. I've had the same ducks come back for three years now. One female even chased a squatter screech owl last year!
They are beautifull yet secretive birds.
Best o' luck! I would put effort into securing posts if that's what you have.

appreciate the advice, both of you. orcuttnyc, no trees around the pond. I think I will simply sink the pole near the pond's edge, on land and will ensure there is a good predator guard in place. I'd have loved to have seen the duck chasing the owl...sounds like you need an owl box now. :)

Speaking of cinder blocks, I have a stack of about 80 blocks in my back yard which are attracting wood ducks. I have an occupied nesting box in my nearby pond, but this morning at least 4 wood ducks (perhaps including my nesting pair) were exploring the cinder block stack. Has anyone experienced anything like this before?