You can use the Water Drop in the Control Center.
Quick Press and release on the flat oval side button (see picture)

Use Control Center on Apple Watch - Apple Support
DO NOT hold the button, as that brings up the Emergency services display.
Then scroll down to the water drop 💧icon
If you have difficulties getting the press and release timing correct, you can adjust the
Settings -> Accessibility -> Side Button Click Speed
Disabling the water lock is done by pressing and holding the Digital Crown (that is the other side button; it is round, sticks out, and rotates; press and hold that until the speakers stop squawking).
How to use Water Lock and eject water from your Apple Watch - Apple Support