Can I lock a watch screen?

Hi

Under the current version of watchOS, with Wrist Detection turned on, it is not possible to manually lock Apple Watch such that the passcode needs to be entered to unlock it again. Your watch will instead lock automatically upon being removed from your wrist.

On Series 2 models, in Control Centre, Water Lock can be turned on by tapping on the water droplet button. This locks the screen to prevent accidental interactions with the screen. The feature is intended to be used during exposure to water, but it can also be used at other times. Turn the Digital Crown to turn off Water Lock.

If you would like to submit a feature request to Apple, then you can do so here:

https://www.apple.com/feedback/watch.html

More information:

Turn on, wake, and unlock

About Apple Watch water resistance - Apple Support

I had this problem and always had to scroll through several alternate watch faces to get back to the watch face I wanted. I partially solved the problem by going into the iPhone Watch app and removing all the watch faces except the one that I wanted. (The others can always be recovered if you want to change.) Now I only have to reactivate the one face on my watch.