Your Apple Watch reminds you to breathe twice a day by default. Here's how you can disable the reminders and stop it from bothering you. After all, if you want to breathe with your watch, you can always open the Breath app.
One of the big selling points of the Apple Watch is that it puts notifications and reminders right on your wrist, encouraging you to do healthy things like move more (by nudging you towards your Activity Goals for daily movement), stand more (if it senses you've been sitting at your desk too long, it reminds you to stand up and move around), and even breathe (a feature added in watchOS 3 that encourages you to take a moment to do some deep breathing every few hours.)
There's plenty of research indicating that taking a moment out of a busy day to do a little deep breathing is beneficial. But if you don't want your watch nagging you to sneak some meditative breathing into your workday, it's easy enough to turn off (although maybe, just maybe, if you're getting mad at your watch for telling you to chill out and breathe for a few minutes, maybe you should take it up on the offer and chill out and breathe for a few minutes).
To do so, open up the Watch app on your iPhone. Scroll down until you see "Breathe" and tap it.
Tap "Breathe Reminders" on the Breathe app's settings screen.
Tap "None" and exit the app to stop reminders.
You can also change the setting to something else. For example, you could select "1 time a day" for just a single reminder.
Your watch will no longer remind you to take a break to perform deep breathing exercises---if you're on a nag-killing bender, be sure to check out how to stop the watch from nagging you to stand, too.