Waterproof iPhone Case
I go fishing quite often and sometimes will be on a boat. I went ahead and bought the iDry waterproof case for the iPhone 4/3GS from eBay for about Aud$30. I think you could fit almost any phone that has a similar shape to the iPhone in it.
It is a pretty big case that probably wouldnt fit in your swimming outfit, but it comes with a handy neck strap. It has a silicon membrane you you can still use the touchscreen. It had a clear back so you can also use the camera.
This effectively makes your iPhone into a waterproof camera up to 6 meters. I will have to put it to the test... Soon.