General note before getting into the Apple Watch:
No watch, not even a diver's watch, is "waterproof" in the sense that it can be taken to any depth of the ocean, no matter how abysmal, and remain intact, keeping out the water however there are certain standard degrees of water resistance. No watch is marketed as waterproof as far as I know; watches are marketed as water-resistant at the lowest level, and water resistant to XYZ depth for higher levels.
What is the overall outlook regarding swimming and the Apple Watch?