I don't think you can make a quilted cover waterproof. My daughter has a LifeProof case for her iphone and it's waterproof up to 6 ft. It stays on the phone all the time. Not cheap, but cheaper than replacing an iPhone.

I know it won't let the phones float but if you vacuum sealed them you might still be able to use them through the plastic and they can aach a float. Just an idea since I don't own one.