The white plastic sheet is for waterproofing. If it's still under warranty have the dealer seal it. If it's out of warranty, just seal it with tape. Others may respond with their recommendations, but I've seen Eternabond recommended for roof repairs. It should be fine for your repair. I've never needed to use anything on our floor or roof.
The attached floor cross section was posted on this forum a while back. I'm not sure if yours is the same.
2011 Sunseeker 3100
1997 Pathfinder Toad