Hello urdone21. I've had my truck parked out in pooring rain all day and when I got a break to wash her down the next day, the bed ribs along with the nipples all over the underside of the heavy rubber mat really does a great job of preventing water from getting completely underneath, and if some did get under by chance, the rubber nipples allow air space for the water to evaporate or run out. I still flip mine over after a truck wash to wipe down the bed and you would be surprised that the water is restricted to the outer bed ribs majority of the time. No more extra time in maintenance aspect than say the amount of time it takes to check tire pressures all the way around.
I took the cheap route and went to the T.S.C. store and bought the 3/4" 4x6 horse stall mat for under $50. cut off a couple of inches.Solid rubber that dosen't move around or let things slide to the back of the bed ,and under the Tonneau cover no one see's it.