Not sure why you've been unable to find any info on the subject, but...
"...the Apple Watch is rated to the IPX7 waterproofing standard.
Under the IPX7 designation, the Apple Watch will be able to withstand immersion in water up to 1 meter for up to 30 minutes. This means that a session in the shower, getting caught in the rain, or washing your hands will not cause damage, but prolonged exposure — such as swimming — would be harmful.
Apple says that users 'can, for example, wear and use Apple Watch during exercise, in the rain, and while washing [their] hands, but submerging Apple Watch is not recommended.' The water resistance rating applies equally to each model of the watch.
IPX7 is a common rating for small portable electronics that are designed for use outdoors, but are not considered rugged. Many handheld GPS units, for example, carry IPX7 ratings."
And of course, there will be people testing those recommended parameters, and there have been videos of people diving in them... I'd stick to the handwashing, rain, shower variety.