Softside Waterbed Mattress Internal Safety Liner. Deep Fill Depth. If you own a waterbed that has foam sides and looks like a regular mattress, then this is the place to find safety liners for the softside style of waterbed. If the safety liner inside that protects the base and foam rails needs to be replaced all you have to do is measure the inside depth of your foam side rails. Choose the appropriate depth: Deep Fill: measures between 7-9 inches deep Mid Fill: measures 5-6 inches deep. Features & Benefits: Install to prevent water spillage in the case of a mattress puncture/seam separation. For your peace of mind. Protect your furniture and floors from water damage in case of a mattress leak/puncture Premium virgin vinyl waterbed liner Fitted tapered style corners. Fits all styles of slant wall style softside waterbeds (Mid Fill & Deep Fill)