The resistance is governed by the standards ISO 2281 (for standard devices) and ISO 6425 (for diving watches). It means that the instrument is sealed against water penetration and leak proof certificate. The value that accompanies the mark determines the static test pressure and must be indicated in bars, atmospheres or, generally, in meters of water depth (in feet for the Yankeedoodles). That is, 50M does not say that the watch can be submerged 50 meters, but that it can reach a pressure of 50 meters of water. It is a unit of pressure, not depth.