When it comes to watches, WR means Water Resistant.

Water Resistancy indicates how well a watch is sealed against water entering the works, once pressure tested. Depending on it's WR rating, the watch may be used in wet environments or underwater to specific depths/levels of pressure.

This refers to water as a liquid - it does not refer to humidity or dampness. Citizen Watches states the following - "Avoid exposing the watch to heat (i.e. spas, saunas and hot showers) as extreme heat will cause some watch materials to expand, thus impacting on the water resistancy capabilities of the watch."

The Water Resistancy rating does not mean a Water Resistant watch was designed for use in the water depths indicated. Example -   A watch with a 30m rating is not designed to be immersed in water, let alone to a depth of 30 metres under water. Moving your watch in the water, as in swimming, exposes it to greater pressure than just the depth of the water.  The minimum rating to immerse a watch in the water is 50m.

Water Resistancy can be compromised as soon as the watch back has been removed, say, to replace a battery. To ensure Water Resistancy is still effective, new seals & a pressure test should happen concurrently with the battery change.

Water Resistancy can also lessen over time, as seals and some parts start to wear. It is recommended that parts such as the crown, seals and other parts important to the water-resistancy of the watch should be replaced every two years to ensure water resistance.

The table below gives the ratings available on most watches, and what suitability the watch has in regards to water resistancy.

* Professional Diving or Saturation Diving require a specifically designed Diver's watch, usually 300M+

No matter the rating, once a watch is wet, it should be dried off immediately. If exposed to salt water, the watch should be rinsed in luke warm water and dried - if you have any concerns take it in to a watchmaker or jeweller.