Wet Wading Waterproof Case/Bag?

I'm looking for suggestions for a case or bag that is TRULY waterproof (and reliably so!) that I can use for wet wading. I need something that will fit in the pocket of my shorts and is large enough to hold an Iphone and my car key. I've heard about using ziplock freezer bags double (the non-zipper type), but I think I'd like something more secure.

I don't want to find out the hard way that something leaked. What say you experts?

John

Last time I was in REI they had a couple options from something that looked like a heavy duty ziploc sized for an iPhone to a hard plastic case that was like an underwater dive housing.
For iphone check http://www.bhphotovideo.com/bnh/cont...=REG&A=details for the car keys well I got nothing.