Plastic under seat vinyl ?

[Jmunk]

I plan to only use enough plastic to cover the foam and wrap a inch under the plywood base and leave the rest of the wood base open to breath. There is a hole in the center of the wood to allow the foam to get air.

 

[Woodonglass]

When I built the Rear Bench Seat for my boat a made the frame and Webbed it. Then I covered it all in vinyl with a couple of Grommet holes on the bottom of the back. Worked really well

 

[Sunken Ship]

My 2000 Larson had the plastic on every seat cushion. The foam will chafe and split like being cut with a knife where the seams are over time-mine did. It doesn't look like your boat has been neglected nearly as bad as mine, but what these guys are telling you is exactly what I found on my project a few months back. I learned there are several different types of foam. One type of foam allows water to run through and will not retain water. I think a few pieces of wood had large holes (about 1 inch) drilled in the wood. Marine plywood has proven to be some amazing stuff on my boat. Good luck with project.