What is the water drop on my Apple Watch?

The Apple Watch Series 2 and later is water resistant and offers swimming workouts. Once a swimming workout begins, the Apple Watch automatically locks the screen. Apple calls this Water Lock, which prevents accidental taps on the screen.

When Water Lock is activated, the green water droplet icon appears on the Apple Watch face. To deactivate Water Lock, turn the Digital Crown to unlock the screen. This will also initiate a process to clear water droplets from the Apple Watch using sounds.

Water Lock and the water clearing process can be manually activated on Apple Watch under Control Center -> Water Lock.