You can swim with the series 6, but the watch isn't waterproof it is water resistant. Constant subjection to harsh pool chemicals can eat away the watch's seals over a period of time.
You can activate the water mode in the watch control center, tap the waterdrop icon. This locks the watch's screen against accidental activation. You exit the water mode by turning the digital crown until the waterdrop disappears from the screen.
M1 Mac mini, macOS 11.2.2; Watch, series 5 GPS/cellular, watchOS 7.3.1; iPad Pro 2nd Gen, iPadOS 14.4; iPad Air 2, iPadOS 14.4; iPhone 12 Pro, iOS 14.4; TV 4th Gen, tvOS 14.; HomePods, audioOS 14.4 and Airport Express w/ Airplay 2