If you have water behind a bathtub liner, it should not cause any damage to the liner itself. The bathtub liner is made out of of either acrylic or PVC. Neither of these materials will be damaged by contact with water. As long as you remove the water, you should not have any permanent damage to the bathtub liner. Other serious problems may arise if water gets under the liner. For example, water in a dark place can lead to mold or mildew, which cause health problems.