No watch can be said to be "waterproof" under federal regulations, only "water resistant" to specific depths. The Apple Watch is water resistant in that you can wash your hands, and probably take a shower while wearing it—some Apple staff claim they do—but they strongly recommend not swimming with one. Too bad.
There really is no reason Apple could not make one pressure proof to a considerable depth except there is a port under one of the bracelet connector slidea. . . which, apparently provides access to the interior. Everything else is sealed.
The SONY Smartwatch 3 (like their Xperia phones) is indeed waterproof, and they call it that.
My current watch says it’s waterproof to 20 meters...that’s what I wish the Apple watch had.
Maybe the power connector and sensors wouldn’t work. It’s probably the port, though.
See ya’,
Ed