If you're thinking about re-building the frames of your seats or possibly re-flooring a boat you would normally be replacing it with marine grade plywood. The only difference in marine plywood and non-marine plywood is it is pressure treated with a chemical that prevents rotting, and the price.
Theres another product out there that goes by the brand name Advantech, its available at most home centers and resembles OSB. The appearance is the ONLY thing similar about these two products, if you have ever used OSB you know it can't get wet or it will delaminate and hold water, the same holds true for MDF, another good product that has it's uses but is not suited for a marine environment..
Advantech is commonly used in the construction industry in subfloors.. To look at it it resembles OSB but this product will NOT absorb water. You can take a 3/4" sheet of advantech cut it into strips and throw it in a pond, if you wait a week and fish it out it will still be straight and true, run a saw through it and you get dry sawdust it absolutly doesn't take on water.
I've used it in the construciton of custom speaker enclosures where normally MDF would be the material of choice.. MDF doesn't shrink and warp like plywood does however MDF will come apart in a marine environment.
Advantech has all the advantages of marine plywood without the warpage and costs less!!
If you're looking to replace some floors or rebuild some old broken down rotting seats I highly suggest you take a look at it.
