You don't want to get water in your watch, everyone knows that. It can be the sweat from your wrist travelling down your stem into the keyless works, it can be the condensation on the inside of the crystal on an old watch that isn't even dustproof, or the salt water from your vacation, maybe you have a dive watch and forgot to screw the crown down.
Or maybe you wore the watch, took it off and laid it on the bathroom counter while you ran a bath or shower. As the air fills with steam, the watch is now cooling, and sucking that air into itself.
Watches in the past bore the marking "waterproof" but they were no such thing, and the word can no longer be used. It is now something that must be accompanied by a rating, as in the case of dive watches and sport watches. What is important to remember is that these ratings are measured statically, that is, motionless. Plunging your arm through the surface of the water while swimming requires a serious depth rating of 100 meters.
Warranties on rated watches are very particular. Hot water will void most warranties so that means no showers no hot tubs. Salt water will likewise void some warranties, so no sea swimming. Activating the pushers on the chronograph will void still other warranties, so unless it is an ISO certified dive watch, no activating the chronograph under water.
If you can screw your crown down, chances are it has a decent rating, but if the pushers do not screw down on a chronograph, that's the Achilles heel right there. Again, it is critical to replace the gaskets in the crown and elsewhere regularly if you want to seriously maintain the rating.
The certified dive watches must be re-certified, contact your manufacturer, because lives do depend on this.
Even a typical 100 meter sport watch must be re-tested every 1-2, and ideally the gaskets in the crown, crown tube, case back, and bezel should be replaced. Most of the higher end makers will do this as part of the full service, or CTR. Without this, do not assume your watch is still water resistant to the rating marked on the watch. Most warranties are not longer than 2 years, so this is usually an added expense if you want to use your watch under water with impunity.
In addition, showering with the watch will erode your bracelet, even if it is Stainless Steel. The pins will get soap residue, dirt and other contaminants on them and this will accelerate the erosion. If a pin fails, your watch hits the ground.
Heat shrinks rubber, so hot water contracts your rubber gaskets, and bam! A leak.
By all means, read your manual if you buy your watch new, it will provide you with all the cautions and ratings of your watch, and you will have many happy years as an informed owner.