It actually doesn't matter at all. It's not a lock, it's just a mechanism to trigger the expulsion of water once you've removed the watch from the water. You can literally just get out of the shower, turn the water lock on, then let it push the water out. There's no need to ensure that the water lock is turned on while it's in the water, it's totally irrelevant.
EDIT: There is a shortcut option for the water lock, so you could create a shortcut to turn it on, then use Siri to run the shortcut. That would allow you to turn it on with just a crown press and voice command.