Just foam. That is the same thing that I experienced. The foam is a closed cell type so it holds the water back really well. I actually shoved a glow rod (for fishing wires in walls) between all three holes and there was no compartment or anything separating that whole space.
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10-17-2012, 09:47 PM #13'08 Mobius LSV, Mods- 1100 lb Rear Sacs, IBS Sac, Center 400 Sac, Dedicated thru-hull intakes and pumps per sac, SeaDek One Piece Mica over Black Roo, EZ Net Cargo Nets, Swivel Drivers Seat, Hot Water Shower, Hinged Wind Block, Bow Cushion Adapter Plate for Drink Holders Access, etc, etc.