I have heard of some using plywood coated in fiberglass resin for water resistance for the floor.
As for the back seat, Im not sure what my base is made of as it is completly covered. I think it is plywood too.
Here are a couple of pics that I have of '96 backseats. They are from two different boats. Note that one has blocks under the seat for support.
1996 Prostar 205 LT-1