Yes you can swim with a watch during the activities such as swimming in a pool or an ocean. 

It has a high resting water resistance under 50 metres so don't go deeper than that . 

If you swim so deep like a sea diver, in that case you should take off your watch because it is quite costly so remember this during swimming time. 

Read More:- https://cleanersuite.com/how-to-turn-digital-crown-to-unlock-and-eject-water-on-apple-watch/