Spoiler Alert: You're going to laugh and ask 'really?'
I've done this approach a number of times, and I'm really not as foolish as this approach may sound.
I purchased a Dry Pak waterproof bag [slightly larger than my iPhone 4S] for under $20 [for examples, see http://www.sportsstuff.com/c/dry-pak_new-stuff ]. First, I just used it for those 'just in case' situations to keep my phone from getting wet. Once [after a few beers/margaritas on a Mexican beach], I decided to take a chance and do some snorkeling with my phone in my Dry Pak so I could get some pics of the angel fish swarming around me. Nervously, I dunked it; then longer underwater, then after a while, I quit paying attention. Now I've done this scores of times without ever getting a hint of moisture on my phone [hint: open/close it out of the water/with dry hands]. I've even taken pictures underwater using my iPhone - not great pics, but nice. I do the same thing when going to football or soccer games in the rain.
Caveat: the Dry Pak is not for anything approaching deep or even the beginnings of rec diving, but it will work for snorkeling and such. I've even dropped mine into the river while fishing, only to grab a mask, jump in the water, and 'fish' it out without any negative consequences.
Second Caveat: notice how inexpensive this approach is?