Is the Apple Watch series 3 Nike really waterproof?

See this, About Apple Watch water resistance - Apple Support

The Apple Watch is water resistant, not waterproof. Apple does not recommend wearing in the shower, nor doing dishes, but the other should be okay.

Technically its water resistant to a certain degree, not water proof. This degree of water resistance however does let it be used for shallow water activities such as swimming in a pool or ocean. Washing dishes and the shower will pose no risk, though Apple recommends against exposing it and certain bands to soaps and other cleaning products.

See here for more details: About Apple Watch water resistance - Apple Support