Water resistant watches are the most common type of watch to find. They are not totally water proof, but are resistant to some degree. It is best to refer to the original company and your manual to determine what kinds of activities your watch is rated for. For example, some watches can be swum with or worn in the shower for a short time, while others are only rated for washing your hands and brief contact in water without immersion. This rating is for your watch as it is in brand new to peak condition. Do not take the ATM number and assume what your watch is capable of. Use the manufacturer’s guidelines to determine what that rating means in real-life situations, such as the ability to do dishes wearing it, a few laps in a pool or snorkeling.
The best way to ensure your watch is in peak condition is to be conscious of your watches limits. Also, when you have your battery changed, it may be a good idea to have the gasket on the back of the watch changed as well. Gaskets can dry out and become less elastic over time, even if you do not regularly get the watch wet. If you are planning to get the watch wet, make sure all crowns (what you use to change the time) and buttons (often for chronograph purposes or to change the date) are in the proper positions. For example, the crown must be closed all of the way (and screwed in, if that is appropriate for your model watch) and the buttons (if there are any) must be in a normal position, not stuck pushed in. Over time, any gaskets involved with these crowns or buttons can sometimes be replaced as well, as a precautionary measure. Also, make sure there are no chips or cracks in the crystal, as this can let water in as well.
You also have the option to test the water resistance of your watch if you take it to a reputable jeweler (that also has experience with watches-not all do) or watchmaker, or send it back to the company that manufactured it.