Bruce; thats not a bad price; for either Plywood or Starboard.
One thing I found when tearing into my seats is that Carver used what looks to be MDF.
I am puzzled, as MDF is generally for interior use only, and will become ruined quite quickly outside. Unless of course, they used an exterior grade of MDF, or the seat bottom isn't really MDF.
When I took the forward seat off last fall, I discovered standing water under the seat, trapped between it and the fiberglass.
So an improvement I am making is to install a scupper to drain the water from that area. To facilitate the scupper, I will create a clearance area on the seat bottom to allow the water to freely get to the scupper.
One reason we are replacing the vinyl in our seats is that after 14 years, even in Michigan daylight, the seats have been UV damaged to the point that the vinyl is becoming brittle and kind of fuzzy.
For me, I am going with plywood, encapsulating them with epoxy as you are considering.
If it doesn't last long, I'll at least have gained more upholstery skills to create a better looking seat the next time.
One thing to make sure of is to have the seat maker put drain holes in the plywood so that any water that finds its way into the seat can drain out of the bottom.