I purchased 3 carseat liners to test them myself. The Britax Seat Saver Waterproof liner, the Summer Infant total coverage car piddlepad and the Summer infant deluxe piddle pad. I put them on our high chairs, poured 1/4 cup water on each of them and waited 5 minutes to see what would happen. The Britax soaked in all the water, but the bottom side was totally dry. The Summer infant deluxe piddle pad came in 2nd, while it soaked in some of the water some stayed beaded up on top of the fabric. None leaked through the bottom. Finally, the Summer total coverage piddlepad came in last. None of the liquid soaked through, it all stayed beaded up on top. So if it was urine, taking a kid out of the seat it would have run all over and still made a mess. No liqud soaked through the fabric so I can't really say if the bottom liner is waterproof or not. We are returning both summer piddle pads and purchasing more of the Britax for both of our cars.
FYI if you are considering the summer piddle pads, they are exactly the same size (though one advertises extra large waterproof protection). The same pattern is used to make both pads. The Britax has the most coverage out of all 3 models. I hope that helps!