Apple's iPhone 6s is not waterproof, though it was engineered well enough that it may be able to withstand brief encounters with water without suffering any damage. The iPhone 6s should not, however, be exposed to water, as doing so may render it inoperable and void the warranty.
When designing the iPhone 6s, Apple added a gasket around the inside perimeter of the device and covered all of the electronics with a waterproofing material. Practically speaking, this caused the iPhone 6s to become extremely resistant to water damage, and multiple tests have shown the iPhone 6s surviving prolonged contact with water. Because Apple does not advertise this fact, though, and it is not officially supported, no one should ever intentionally submerge their iPhone 6s.