Water lock
What happens is I forget to start the swim work out or start the water lock
Apple Watch Series 4
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The session won't be recorded and your screen will be active during the swim. You won't be able to expel water from any of the openings on the watch, but you likely won't damage the watch. At a minimum, I'd turn it on after the fact if you forgot to turn it on before starting the swim and then turn it off to force air to push out any water.