Apple Watch's wrist raise feature turns on the screen every time you lift your wrist, showing you the time and any other notifications that may have popped up. It can be convenient, sure, but it can also drain your battery.
To turn this off, navigate to the Settings app on either your watch or on the Watch app on your iPhone. Go to Settings, then General, then Wake Screen.
Here, you can disable the Wake Screen on Wrist Raise and Wake Screen on Crown Up features by toggling the green button to the right of each option.