Can I use the water lock on my apple watch series 3 after I swim?

I just had a quick question about my Apple Watch. I have been using it quite often and I always get stumbled when it comes to swimming and when I should use the water lock. I always turn it on when I go swimming but wasn't sure if that was necessary since it's water proof. So my question is: When I go swimming, should I turn on the water lock before I go swimming or just after to remove water?