Apple Watches are water resistant to a degree. ‘Waterproof’ isn’t the best term to describe it because that implies that water cannot entet it at all.
The documentation of your Apple Watch and the back of its case will tell you how many meters it it can be submerged before its water seals fail.
For example: The Apple Watch Series 8 says ‘WR 50m’, which means that the Apple Watch Series 8 can be submerged in water up to 50 meters deep.
The water resistance of Apple Watches varies with their version and tier. The first generation of the Apple Watch Ultra is water resistant up to 100 meters deep.
Finally, bear in mind the fact that age, wear, and cracks can easily compromise water resistance. A cracked watch, for example is not water resistant.