Here is a product that is safe and non-toxic for waterproofing walls and even floors. They have various products for different needs. Hope this is helpful, good luck!
Product Details
a stainless steel film will be easy to keep clean and won't do much damge to the wall when removed. http://www.amazon.com/Instant-Appliance-Stick-Stainless-Steel/dp/B00BAUPS9M