I know a guy who build a floating dock from wood and underneath were 5 gallon buckets upsidedown with the lids sealed on to keep them full of air.
He built the dock floor upside down like a shallow box, the buckets were screwed on throught the bottom into the underside of the floor and sealed with silicone. Lids sealed on and then he just flipped it over into the water.
It's pretty stable.

We have a raft made with barrels to hold it up---have seen the same thing done on rivers where the piers have to go up and down when the river rises and falls.

Hi Poppy_luv,

I work for a marine construction company and a better suggestion for your walkway besides the 5-gal buckets, would be floats. They are plastic and can be about 3' - 4' wide and about 5' - 6' long. They attach under the decking that you build. They would be more stable I think to walk on. Or is you want to sink a post (piling) you can jet it thru the muck with water (hose attached to a rod). Hope this gives you some ideas.
Donna in Jacksonville

dock blocks are cool but the pails are free.:)
I would use the blocks as well if money is no object.
Her bridge/floating walk is 40 feet long.
3 18 foot treated 2x6s (2x8s would be better) would create a ridgid rail on each side. If the deck was nailed to that, maybe 4 ft wide treated 2x6.
You would only need two posts one on each side in the middle of the pond.
Heavy cement on top of a steel car rim should work with the posts attached.
Leave the posts long so if the footings settle you can use a come-a-long to jack the walk up the post and refasten.
Using cement footing on each end keeping this walk above water should be no problem.