Is my Apple Watch waterproof

No. Apple does not guarantee anything to do with water damage. As has been pointed out the Apple Watch is water "resistant" but not "waterproof". If you do go swimming with your Apple Watch and it becomes damaged your warranty will NOT cover the damage. To my knowledge the only watches certified as waterproof are professional diving watches. That being said you should be okay with swimming with your Apple Watch BUT it is not guaranteed to not sustain water damage. In my experience NO consumer electronic device has such a warranty.

It is water RESISTANT, not waterproof. From:

https://support.apple.com/kb/SP778?viewlocale=en_US&locale=en_US

Apple Watch Series 3 and Series 4 have a water resistance rating of 50 meters under ISO standard 22810:2010. This means that they may be used for shallow-water activities like swimming in a pool or ocean. However, they should not be used for scuba diving, waterskiing, or other activities involving high-velocity water or submersion below shallow depth.