Construction Ideas?

[Duck Butter]

We are going to be making a 200' boardwalk and need ideas for the materials. The boardwalk goes over a wet area and will flood occasionally. We currently have a raised boardwalk there with posts cemented in and pressure treated wood, etc but the ground is too mucky and the boardwalk tends to warp over time due to the posts rising out the ground.

I am thinking something that will float when needed but will be on the ground most the time. Styrofoam is out the question because the rats and squirrels would have a field day with it

Do any of you know of any material or some sort of company that makes something like this? Thanks