The rate of water resistance of Apple Watch is IPX7. It means that actually the watch is not water-proof, and Apple does not recommend to swim with watch on or to immerse it in water. But the published manual contains recommendation to rinse the watch under the tap from time to time.
If the digital crown of your watch stopped to rotate as it rotated right after purchase, the reason can be the dirt accumulated between the case of watch and the crown. In this case Apple recommends the following:
1. Switch off Apple Watch and disconnect it from charger.
2. Remove the wristlet if it is made of leather.
3. Hold Apple Watch under low head of warm fresh water from the tap within 10-15 seconds. Do not use soap and other washing liquids.
4. Turn and press the digital crown while water flows between it and the case of watch.
5. Dry Apple Watch with non-abrasive lint-free tissue.
Though, despite of all recommendations from Apple, there is a video showing immersing of the watch in water, and the watch does not break down after 15 minutes in water.