The water resistance isn't listed on Google's official Nexus landing page, but it is listed on Motorola's official Nexus 6 page. Though, under the Water Resistant heading, Motorola's page simply says "yes". There is no IP rating given, because unlike other devices that can handle being submerged in water, the Moto X and likely the Nexus 6 are only perfunctorily water resistant. This means you can splash some water on it, or use it in a light rain without much fear of damage, but anything more than that and you're asking for trouble.
Still, it is nice to know that even a basic level of protection has been added to the device. It helps ease your mind a bit, and adds a tiny amount to the premium feel of the phone.