To test wether a bag/enclosure is water-proof should be easy .. Use CoCl2 (or similar water-detection chemicals) or (rather more simple) a toilet paper to test for water after e.g. getting some air in the bag, closing it, and pushing it under water for some time. And/or put it in a steamed bath for some time - if water is on the inside it is unusable (eaven when using two bags). If no water can be found, it should be okay - don't try scuba with it.
Possible bags perhaps:
* zip-lock bags or other "sealed" bags
* camera-bags e.g. for scuba-diving or snorkling (they are expensive though) (if you can take a picture through it, it should be transparent enough to read through
) - these could even be used to read lying on an inflatable mattress on a pool.
* outdoor-equip for a PDA/Handy/Passport/...
* etc etc etc
Make sure the enclosure is waterproof and you can use the buttons without opening the bag - Murphy tells us, that you *will* drop the device into the water as soon as you open the bag.
If your device is flooded: Turn it off and let it dry completely before even thinking of using it again! If possible: Open the device.