What should I do if my Apple Watch gets wet?

Nothing. Apple Watches are water resistant. Just dry it off.

click here ➜ About Apple Watch water resistance - Apple Support

How did you put water "in" your watch?

Did you mean "on" your watch?

If you mean on, then wipe the water off.

If you mean in, try the old trick of putting it in a bag of rice and waiting a few days for it to dry then try to power it on, you might be purchasing a new watch.