I frequently use a "dry bag" to keep my phones from getting wet. You can search them online. I use mine when I kayak, when I bike, and when I go walking and the weather looks like rain. They come in multiple different sizes. this is the brand that I have. I like it since it comes with both a nylon "necklace" that I can wear around my neck and tuck down my shirt/vest when kayaking so I don't lose it, and the armband when I am biking or walking. the little nipple on the side is to extract the extra air so it is basically vacuum sealed. thus, if the seal is broken, it's obvious when you try to seal it. Simple, easy to use, doesn't void the warranty, can still use it when it's in the bag. I paid around $35 for mine. Hope that helps. I saw the listing on ebay for the little bags that go over the phone, but when I do an internet search regarding the company, all I find are more "ebay" listings. Not so sure about that, but in theory, sounds like a great idea. but if you're into heavy duty water exposure, go with the dry bag!
Dry Case Waterproof Case : Product Description, Reviews and Information