A watch that is considered water resistant may come in contact with water but only to a certain extent. Most watches have a water resistance rating engraved on the case back. A water-resistant rating given by the manufacturer has been based upon a set of very controlled conditions in a laboratory. However, everyday conditions and aging of the gaskets will decrease the watch’s resistance to water over time. Please keep in mind that once a battery has been changed, the seal protecting the watch from water will no longer be effective in protecting your watch from the water. We strongly suggest that you always work well within the parameters of the manufacturer’s recommendations and have your watch tested at least once a year, or after a battery change, to ensure that water will not enter the watch case. We are able to perform a water resistance test for watches that are suited for underwater use as stated by the water resistance rating of a customer’s watch.

Real Life vs. Laboratory Conditions

When a watch is tested by the manufacturer, it is done in a laboratory under controlled conditions. However, real-life actions will produce completely different results. Water temperatures from a hot-tub or a hot shower can slightly expand the shape of the gasket seals. Gaskets are also known to expand and contract when a watch is taken from the hot water and immediately put into cold water (ex. coming from a hot-tub and immediately going into a pool).
Rapid changes in pressure such as diving into a pool can also stress the gaskets for a fraction of a second. If the gaskets are not up to specification they may rupture and cause the watch to take in water. As the watch ages, the seals begin to erode & will no longer maintain the same water resistance levels. 

Water resistance on a watch is obtained by 3 important factors:

1. Case Back - Case back refers to how the back of the case attaches to a watch. Snap-on case backs are considered to be the least water resistant. The slightest deformity in a case or gasket will allow water to penetrate the case. Generally, these watches will have a water resistance of 30m (99ft) – which allows for light splashes of water (ex. washing hands) but not for full immersion in water. Case-backs attached with screws are the second level of water resistance. When a case back is attached with screws, the case back will have a much tighter seal on the watch. However, any deformity in the gasket will allow water to penetrate. These watches generally have a water resistance of 100m (330ft). This allows for light swimming and immersion in a pool. Screw-in case backs are threaded and the case back acts as a large screw. This creates an even seal by using the threading of the case back. The gasket in this situation will serve as a secondary seal. Diving watches with water-resistant ratings greater than 100m (330ft) will usually have this type of case back.
2. Crown - The crown on a watch is used to adjust the time.  The crown is attached to a stem which is attached to the movement through the hole in the case’s edge. The crown is the single most important factor to ensure water resistance. The weakest link in a watch for water to penetrate is the crown hole. Water can enter through the crown hole when the gasket has been distorted from stress. Water will also enter the case when the crown is not fully pushed into place. Screw-down crowns are the most water resistant because the crown acts like a threaded screw. The crown screws into the case and acts as an additional seal to the watch. A screw-down crown is essential for a watch that is intended to be submerged in water. We do not recommend swimming with a watch that does not have a screw-down crown. Please note that a watch with chronograph pushers, or a digital watch with multiple button functions, should never be pressed when the watch is submerged in water. 
3. Gaskets - Gaskets, also known as “O” rings, are made of rubber, nylon or Teflon. They form watertight seals at the joints where the crystal, case back and crown meet the watch case. If the watch is a chronograph, the chronograph pushers will also have gaskets. Gaskets eventually erode and break down over time which diminishes the water resistance of a watch. It is important to test your watch once a year for water resistance. Any competent watchmaker should have the necessary equipment to test the watch.

Water Resistance Testing Method

During a water resistance test, we place a watch into a chamber that is half filled with water and air. We increase the air pressure inside of the chamber and the lower the watch into the water. Once the watch is completely immersed, we release the air pressure inside of the chamber. If bubbles come out of the watch it means that air seeped into the watch prior to immersion and the watch is not water resistant. We are able to perform this test to help us locate any water leaks in the watch case and repair/replace parts to ensure the watch is water resistant.

Water Resistance Guide

When buying a watch, it is important to understand the depth rating as not all watches are suitable for showers or swimming pools. (Please note: Under no circumstances should a watch be exposed to saunas or hot tubs. While a watch may be able to handle a certain amount of feet of water, the steam of a warm area can cause the gasket to expand and let condensation into the case.)