This would seem to be a perfect application for a Pin Foundation:
http://pinfoundations.com/boardwalks.htm
Looks like they have experience with wetlands.
You should review this document if you are interested in the slab also being the foundation itself:
http://www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdfhttp://www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdf