Because it is a Luftwaffe map, it is very likely intended for use in dropping air mines and pressure / magnetic mines in particular. For this the water depth is relevant as these mines do not work beyond a certain depth of water. Given that the map centers on Sheerness, a major RN base, this use is very likely. Why it is waterproof escapes me. Possibly it is one of those peculiar German quirks of being thorough that casued its manufacture in that manner.