None of Apple's iOS devices are waterproof. The closest waterproof Apple product is the Apple Watch, which is listed as "water resistant". Several third-party manufacturers offer waterproof cases for the iPhone 6 and 6 Plus. These cases should work with the iPhone 6s and 6 Plus since they feature the same basic form factor, however you may want to check with manufacturer first since there is a slight difference in width and depth. Third-party case makers will also be releasing waterproof cases specifically designed for the iPhone 6s and 6 Plus in the near future.
Is the iPhone 6s / iPhone 6s Plus waterproof?