Aquatic Custom Tile - Ceramic Swimming Pool Waterline Depth Marker " 1.8 M " Smooth Finish, 5-inch Font, These are the best quality on the market, directly from the manufacturer. All of our depth markers are screen printed onto the finest European frost-proof tile. They are then kiln-fired to over 1500 degrees to ensure the print is permanent. This process guarantees that they will never peel or fade.