One of the best waterproof iPhone 6 case manufacturers is Lifeproof (now owned by Otterbox), which has a reputation for delivering consistently dependable iPhone cases.
The Fre has a scratch-resistant touchscreen cover and can also be submerged in up to 2 meters of water for up to an hour.
For waterproof cases, Lifeproof is tough to beat on usability and style. You can pick this one up in a variety of different two-tone color combinations. It will also protect your iPhone 6 from snow, dirt, and drop damage.