What is the different between water resistant and water proof?
I have owned two LUMINOX Navy Seal dive watchs and both failed due to water incursion into the internals after about a year or so. With no way to get repairs of service, these are throw away watches. Water resistant means it could get rained on. Water proof means that it performs as advertised; forget that!
These terms refer to the degree to which water is prevented from entering the watch. Water resistant usually means that water is prevented from entering up to a certain point (sometimes almost completely). On the other hand, waterproof means that water is completely prevented from entering. Hence, a watch might be water resistant to 300 feet versus water proof.
Water resistance is a measurement of resistance to intrusion of water into the watch's internal mechanisms. Because the pressure of water increases by about 1 atm every 10m in depth, the force exerted on the watch's seals gets greater and greater until water is eventually able to force its way in. Although there is no such thing as a watch that is truly "water proof," e.g. completely impervious to water intrusion, watches that are marketed as being "water resistant" tend to be engineered to withstand the pressure of a casual exposure to water, such as in a bathtub or the shallow end of a local swimming pool, while those marketed as being "water proof" are designed to hold up to a specific dive depth. Other types of environmental exposures, such as sand and sun, will wear down the watch's seals, so the depth at which the watch will withstand water pressure will degrade over time, depending on how well the watch is taken care of.