Water Resistance is an often misunderstood topic. When I was just starting out I figured diving watches were 'waterproof'. A jeweler quickly corrected me and explained the concept of water resistance. Watches are never rated as waterproof, mostly because there will always be conditions that have the ability to flood the watch. Sure, for some fine diving watches, you'd need to burrow halfway to the center of the earth to find such conditions, but as these are foremost watches meant to be tools, performance metrics are very important.
Then there is the confusing matter of Meters/Feet vs Bars/Atmospheres. A 'Bar' or 'Atm' is a measurement specifically geared towards pressure. Luckily, as near as makes no difference:
So a watch rated for 10 Bar is the same as 330 feet or 100 meters. Below is a chart that shows what you can expect your watch to handle based on its depth rating:
If you watch doesn't specify, or is a vintage piece, for the most part these fall into the 'no water' category. You can also look for some tells of whether your watch is meant to be waterproof, but use these only to confirm they are built to be watertight, not that they necessarily are until you get your watch pressure tested by a professional watchmaker. Here are some things to look for to decide if a watch is engineered to handle water submersion.
First, screw-down crowns with little rubber rings or gaskets where the threading shows. One of the biggest innovations in water resistant watches was this very thing. Older watches could easily be flooded because water can go right in where the crown and pushers enter the case. With screw down crowns, the crown itself remains usable, but keeps water out when it is screwed down. For chronographs, all the pushers and the crown are typically screwed down so there are no exposed entrances into the watch. It is recommended to keep these screwed down when not in use to keep out other particles as well. See below on the Tudor Heritage Chrono:
Second, a thick crystal over the watch face. The crystal is a major part of making sure things don't get into your watch casing, as crystal is a lot less stable than metal. Watchmakers began to install thicker crystals on their diving watches to withstand the pressures which come from the deep depths. Rolex famously made a very rare watch called the Deepsea Challenge, capable of withstanding the pressures at the bottom of the Mariana Trench, the deepest known point in the ocean at 39,370 Feet. Or 7 miles.
The original prototypes had a massive crystal that looked like a crystal boob. The more recent version still uses a very thick crystal, but at least looks like a watch due to modern technology. Check out the construction below:
Finally, a watch truly meant for deep water applications (or aspirations thereof) will have a helium escape valve. During prolonged deep-water dives, Helium (He), the smallest particles found on Earth, often sneaks by the watch's gaskets and other security measures and build up inside the watch casing. While resurfacing, if not released, the Helium depressurizes, which can cause the crystal to pop off the watch.
In sum, if you have a highly water resistant watch (100m+), don't be afraid to swim, snorkel and shower with it. The reason that many of these watches are so expensive is the extreme over-engineering and thought that goes into making it a purpose built instrument. The deep end at the country club pool isn't going to give it anything to worry about.
That said, before you submit any watch to significant water exposure, make sure you get it pressure tested whenever it needs a service. If you use it often for deep dives, I'd recommend you check at least once a year to make sure it is staying properly sealed and that the gaskets aren't getting worn out. Its a cheap and fast process that will give you peace of mind and is nothing compared to ending up with a fish tank on your wrist.