Can I swim in the sea with my Apple Watch 7?

Yes,see here;

Swim with your Watch

I wouldn't. Water damage isn't covered by the warranty. You never know if the water resistance has been compromised.

M1 Mac mini, macOS 12.4; Watch, series 5 GPS/cellular, watchOS 8.6; iPad Pro 2nd Gen, iPadOS 15.5; iPhone 12 Pro, iOS 15.5; TV 4th Gen, tvOS 15.5; HomePods, audioOS 15.5 and Airport Express w/ Airplay 2

Also: About Apple Watch water resistance - Apple Support