Cascade Designs makes a number of different waterproof E-cases for various devices, including the iPhone 4S and iPhone 5 (as well iPads and Android devices). They have an IPX7 certification rating, which means they meet the standard of withstanding submersion in 1 meter of water for 30 minutes.
The good thing about this type of SealLock case is that it isn't so tied to a specific phone model. That means it will most likely work with future iPhones as well as iPod Touches and other phones. Out of the water, you can also operate the touch screen through the plastic cover and the back of the case is translucent on top so you can take pictures or shoot video.
Note: A higher-end version of the case adds a jack for headphone use for $10 more.