The actual seat material is the same as the rest of the seats and will not soak water any more then any of the other Mustang seats. The problem is all of the stitching on the Tuck and Roll or any other after market seat with a lot of stitching.
Applying Carnauba wax on the stitching does take care of this problem.
Mustang suggests spraying the seat with "ProtectAll" to block the stitching holes and prevent water seeping into the seat. This spray protectant has carnauba wax in it and by spraying the seat with this spray, letting it sit for 5 minutes then wiping off and buffing seat, the seams and stitching holes are blocked by the wax and it does a good job of preventing or greatly reducing the water that can seep into the seat.
Hope this helps.
Rick