MDF or HDF is great for some things, but I would never use it for something exposed to water, no matter how it is rated, and in any event I would use neither in this application (I'm first and foremost a woodworker).
First, even if rated "water resistant", that is much different from marine grade plywood. MDF and HDF will eventually absobe water and because it is essentially glued sawdust, it will swell and start to disintigrate.
Second MDF and HDF is much heavier for the same thickness- much more dense.
Third, MDF and HDF (MDF moreso) is much more flexible than plywood.
MDF and HDF are a wonderful materials. I have used both in projects, but I wouldn't use it for this.
As an asside, I would rather sand plywood than plane it to reduce thickness. I don't know how wide the pieces are, but a home woodworker is unlkely to have a planer over 14 inches (you may find 20), but an open sided drum sander will be able to do a wider piece, will be easier to remove smaller amounts, and will leave you with a very nice surface.