Swimming with a touchscreen on your wrist can mean that the water sometimes accidentally touches the screen and selects things. To solve this, Apple created Water Lock, which locks the screen and prevents accidental touches.
The MySwimPro Apple Watch app will automatically engage water lock when you start a swim workout. To access the menu any time thereafter, you will need to unlock water lock. To do this, simply spin the digital crown until you see the unlocking animation and hear the beeps.
If at any time you want to engage water lock again during the swim workout, force touch the menu and select the Lock button.