My Apple Watch Series 6 is damaged by the water. What can I do?

You're right, Apple Watch is NOT waterproof. Apple never said it was. Apple has always said Apple Watch is water resistant. That's a significant difference. Your subject asks what you can do? Since you've already learned the watch is damaged beyond repair, you recycle the watch and buy a new watch.

No purpose in us debating this here. This is a user to user only forum. No one here works for Apple. Apple never said as you suggested above that Apple Watch is Waterproof. That would indeed be false, but they specifically say Apple Watch is Water Resistant. That's a significant difference.

Either way, you need to take your grievance to Apple directly. There is nothing any of us users can do to help you.

Apple also stated:

…above (and more) from: About Apple Watch water resistance - Apple Support