John Gruber points out this bit from the bottom of the Apple Watch Health and Fitness page:

Apple Watch is splash and water resistant but not waterproof. You can, for example, wear and use Apple Watch during exercise, in the rain, and while washing your hands, but submerging Apple Watch is not recommended. Apple Watch has a water resistance rating of IPX7 under IEC standard 60529. The leather bands are not water resistant.

That means it’s rated for one meter of submersion for up to thirty minutes. I’d love to see Apple add this feature to iPhones in the future.