Apple

So while it’s true it’s fine to go swimming in shallow waters (which I think Apple defines as 5 meters) with an Apple Watch, I would avoid showering with it for 2 reasons:

1. Water pressure. The seals in the Apple Watch can keep water out at “normal “ pressure, but the water coming out of your shower head is hitting the watch at a speed that can exceed the force the seals are designed to repel water. Hit the watch with a stream of water form the shower head at the wrong angle and water can get inside.
2. Surface tension. Soap works by combining with water molecules to lower the surface tension of the soapy solution, which allows it to penetrate things better than non-soapy water cannot- stubborn dirt, grease, the surface of your skin, and yes, the seals in your Apple Watch.

That said, I shower with my Apple Watch Series 5 every day and it’s fine. (Sort of- it reboots randomly, but I think that have more to do with the incompatibility of the latest watchOS with older Apple Watch models, since the behavior started right after I upgraded to watchOS 10)

I plan on replacing my watch next year anyway. If my watch was newer and especially if it was still under warranty, I would take it off before getting in the shower. They do have indicators that tell repair techs whether or not water penetrated the case and if so, they will refuse to repair it unless covered by AppleCare+.