apple watch waterproof

The Watch is water resistant not proof. Yes you can enter the sea. But wash off any salt. Personally I would not wear it in near water

https://support.apple.com/en-gb/HT205000

I have worn mine many a time in the sea and the pool without any effects (so far that is).

I just rinse it off well under cool water when done.

Be forewarned that the warranty does not cover water damage.