When a watch say its water resistant to 3 ATM of pressure can you take a shower with it on or go swimming with it on?

Answer 1

A watch that sates it's water resistant to 3 ATM of pressure is almost always safe to shower in and you can swim in it as well. Although there are a couple of caveats. First "1 ATM" or one atmosphere is equal to the pressure of water at 33.8995 feet deep when the water is 39.2 degrees Fahrenheit. The temperature of the water will not make any difference if you are wearing the watch, since you will not be swimming in 39 degree water for very long unless you are a diver in a dry suit. If you were you would probably be wearing a watch water resistant at a much greater depth such as a Rolex. As you probably know these watches are very expensive. (A decent diving model starts somewhere in the range of $5,000.00 and goes much higher.) A Luminox Navy Seal Model only costs about $300.00, and it is water resistant to 200 meters (656 Feet). This is the same watch issued to military divers and it has proved very water resistant while swimming and showering.
Basically your watch "states" it is water resistant up to 3 ATM or around 100 feet. So I would say as long as you are not Scuba diving scuba diving, and the watch is made by a reputable watch maker that warranties its product you will be safe showering or swimming with your watch on.
Also, there isn't any watch to my knowledge that is completely water proof since they all are man made and have seals that can leak, hence the expression, "water resistant". There are different innovations in modern watch design that are used to make a particular watch more water resistant than another such as double "O" rings around the stem, screw down stems, a cap that screws down onto an "O" ring over the stem, screw down plates onto seals around the crown, and the same on the back of the watch.
On a side note, ATM, or atmospheres, is a measure of atmospheric pressure which is done in several ways such as Bars, inches of Hg (Mercury) on a weather gauge or Pounds per Square Inch which is about 14.7.