A lot of people have pretty good success with the rubber mat but I chose to go the bedliner (Line-X) instead. As far as water in the bed, the FWC campers have 1x3 stingers that run the length of the camper floor to allow air circulation to aide drying. Personally I think the bedliner offers a more stable surface for the camper. I haven't experienced any movement or shifting of the camper. More importantly I haven't experienced any loosening of the turnbuckles that some have.
The turnbuckles loosening happened on my dads 04 Tacoma with a FWC (has a steel bed). We just finished fixing it. In his case, it was not the rubber mat, it was that the rear of the bed buckled and the front of the bed tore open. The camper was attached with eye bolts and 2.5" fender washers. the bed is now plated with 1/4 and 3/16 plate and the camper is attached with 5k lb d rings welded to the plate.