From the Apple Watch User Guide > Safety, Handling and Support (with my emphasis added):
"Medical device interference
Apple Watch contains components and radios that emit electromagnetic fields. Apple Watch, some of the bands, the Apple Watch Magnetic Charging Cable, and the Apple Watch Magnetic Charging Case contain magnets.
These electromagnetic fields and magnets may interfere with pacemakers, defibrillators, or other medical devices.
Maintain a safe distance of separation between your medical device and Apple Watch, its bands, the Apple Watch Magnetic Charging Cable, and the Apple Watch Magnetic Charging Case.
Consult your physician and medical device manufacturer for information specific to your medical device. Stop using Apple Watch, its bands, the Apple Watch Magnetic Charging Cable, and the Apple Watch Magnetic Charging Case if you suspect they are interfering with your pacemaker, defibrillator, or any other medical device."
https://help.apple.com/watch/#/apdcf2ff54e9