Not waterproof, just water-resistant. A pair of plastic zip lock bags, one inside the other, both zipped shut, will make any phone resistant to a few feet. (Check with the bags you use with no phone in them to see how much pressure they'll withstand [that's the deciding factor - how much water pressure it takes to force water into the bag] before putting a phone in it - then use half your test depth at most as your maximum. IOW, if you get a leak at 6 feet, never take the phone down more than 3 feet.)
If you don't need to use any controls, you can make a box out of 1/8" plexiglass, with one face sealing against an O-ring set into the four edges. Even a few pounds of pressure against a lubed O-Ring can withstand a lot of water pressure.
As for a ready-made case, Lifeproof cases are good to 2 meters, most other cases are good to only 1 meter. The url=http://www.amazon.com/FRiEQ-Universal-Waterproof-Outdoor-Activities/dp/B00AW2QT8I/ref=sr_1_4?s=wireless&ie=UTF8&qid=1433388987&sr=1-4&keywords=waterproof+case]FRiEQ[/url] claims to be waterproof to 30 meters. I don't know, I've never tested it. (I haven't been 30 meters down in any water in decades.) It'll work with the phone, but it's not the prettiest thing, so unless looks are important, it may be your best bet. (And it's less than $10.)