I would suggest you follow the steps bellow to turn on water lock on you Galaxy watch 4:
Your Samsung Galaxy Watch4 can be made water-resistant by turning on the Water lock mode. This keeps you from accidentally triggering your watch in water and, upon turning off, ejects water. To turn on Water lock mode, follow these steps.
Go to the Home screen. Swipe up.
Scroll down. Choose the settings symbol.
Scroll down. Choose Advanced features.
Choose Water lock.
Turn on Water lock.
Water lock has been turned on. To turn off water lock and eject water, press the Home key until a sound plays.
The Samsung Galaxy Watch4 will automatically start ejecting the water and then stop.
Scroll down. Follow the onscreen instructions. Choose OK.
Water lock has been has been turned off. Return to the home screen.
