We've played with waterproof cases in the past and so far they have all worked well. This kind of waterproof case is becoming increasingly popular with the enormous amount of touch screen devices being released.
The Dry Case for Mobile devices is basically an all plastic enclosure. Your device sits in the transparent plastic and the top of the case has a clip mechanism for keeping the water out. It's easy enough setup and use and most waterproof cases work on the exact same principle.
Because this is designed to fit a variety of mobile devices you can have one Dry Case for the whole family to take on holiday. But one flaw in it being so roomy causes the devices to move around a great deal which in turn makes the device hard to use. However Dry Case have solved this problem by having a valve on the outside of the case which you attach the hand pump supplied within the box upon and suck all the air out of the case. That way you can use your device with a lot less hassle. But trying to remove your device after doing this is quite hard as it sticks to the airless interior of the case, if you could pump air back in, it would make this a lot easier.
The case also has a waterproof audio jack incorporated which were hesitant to use with our standard non waterproof earbuds and headphones but luckily we have a pair of Dry Case earbuds to test this feature with and it works in and out of water. Another addition which comes in the box is a Velcro wrist strap that attaches itself to the back of the case.
In our tests we ran a tap over an iPhone 4S and the screen when gradually submerged under water was mostly usable. The problem we ran into when we tested the case under running water, was that it activated the multi touch display, so the device would scroll and launch things we didn't command it to do. But when used at the beach and submerged entirely in water this problem didn't occur, instead the screen isn't at all usable. So your iPhone is protected but you can't actually do anything underwater like take photos. The depth you can take this case under is however very impressive, with it being able to handle a 100 feet for a one hour duration.
Of course things like iPod Nanos and devices with physical buttons that can be within reach when in the case would work fine completely submerged underwater.
Overall the Dry Case for Mobile Devices is in one hand a disappointment but it does undercut the competition in price. So if you're not bothered about using your touch screen devices underwater and just want to use it around water with the reassurance that it is protected, then this is worth a consideration.
You can purchase the DryCase from http://www.typhoon-int.co.uk/