My Apple SE watch turn black due to swimming

The Watch is not waterproof, it is water-resistant. If you subject your watch to harsh chemicals its water-resistance can be defeated. And it will stop working.

About Apple Watch water resistance - Apple Support

M1 Mac mini, macOS 12.2.1; Watch, series 5 GPS/cellular, watchOS 8.4.2; iPad Pro 2nd Gen, iPadOS 15.3.1; iPhone 12 Pro, iOS 15.3.1; TV 4th Gen, tvOS 15.3; HomePods, audioOS 15.3 and Airport Express w/ Airplay 2

Hello tejinder144,

Thanks for choosing the Apple Support Communities. If we understand correctly, you are unable to power on your Apple Watch. We can understand why you'd reach out for a solution.

Here is a resource that can help.

If your Apple Watch won't charge or it won't turn on - Apple Support

Cheers!