I have used it once on my Apple Watch to try it out, was not used in water though so just some vibrating if memory serves me well.
After a brief water merging of water and my iPhone XR I am not all that keen to trust Apple and any supposed water proof. I know the XR is not waterproof but they made a pretty big deal about the water resistance that has not lived up to the hype in my case. Not willing to take the chance with my watch.
“If you are neutral in situations of injustice, you have chosen the side of the oppressor. If an elephant has its foot on the tail of a mouse and you say that you are neutral, the mouse will not appreciate your neutrality.” Bishop Desmond Tutu