WHAT IS A WATER RESISTANT WATCH

Water resistance is the watch case’s ability to prevent the water from entering inside the watch.
Water Resist has replaced the now obsolete term of Waterproof, as the  latter is very absolute and does not properly identify the pressure a  watch can be exposed to.
The term Water Resist (WR) is followed by a number indicating the  pressure a watch case can sustain. This number, according to the  manufacturer’s choice, can be either in Metres (m), Feet (ft), Bars or  Atmospheres (ATM).

The units of meters and feet indicated, are a pressure rating and NOT the recommended depth of use.

These ratings take into account the combination between motion and depth.
Since a watch is not static when worn on the wrist, pressure applied to a watch dramatically increases with motion.
In this respect, if you dive in a pool to a depth of 2.5 metres the  pressure a watch will be sustained to will not be 0.25 atmospheres.

Below is the definition for the water-resistance categories. Use your watch accordingly.

Note that one atmosphere (ATM) or Bar of pressure, is equivalent to the  pressure exerted on a static object lying in a depth of 10 metres. Pressure multiplies with motion.

WATER RESISTANT 30m: The watch is safe for accidental contact with splashes of water.
WATER RESISTANT 50m:  Suitable for swimming on the surface of the sea.
WATER RESISTANT 100m: Can be used for snorkeling.
WATER RESISTANT 200m and more: Suitable for scuba diving.

Water Resistance protection is not a permanent state and a watch needs periodical maintenance by replacing all rubber seals, followed by a water resistance test  to ensure the watch's impermeability to water.

In order to preserve the watch’s water resistance: 

•  Do not wear the watch in the bath as the alkali contained in soaps can damage the rubber seals. Also take into account that the water is coming from the shower head with a certain pressure.
• Do not use the watch in a hot tub as the high temperatures can damage the rubber seals.  Do not expose the watch to sudden changes of temperature by going from a hot tub to cold water. This can make the different materials to contract and allow water to penetrate.  
• Always rinse the watch with clear tap water after every use in the sea. Salt is very corrosive and can damage both the rubber seals as well as the watch case. Even stainless steel!  
• Never use the push buttons underwater or when the watch is wet. You are actually pushing water inside the watch.  
• For divers’ watches with screw crowns, make sure before each dive that the crown is properly screwed-in.