Update: Apple did not announce an iPad Air 3 on March 21, 2016, like expected. Instead the company announced the iPhone SE and 9.7-inch iPad Pro. Neither device is waterproof or water resistant.
Recent rumors have suggested that all of Apple's upcoming iOS devices will feature an IP rating for water and dust resistance. At this time none of Apple's devices are "waterproof," but the Apple Watch is listed as "splash and water resistant" on Apple's website. It is unknown if the iPad Air 3 will be Apple's first water resistant iOS product, but we should know more in March when Apple is expected to hold its first big keynote of 2016.