I've been playing around with GPS Kits for my iphone. I bought a bike mount for my phone and I've been using the GPS on it as well as on the trail. So far it's been pretty reliable. I am currently looking on how I will safely mount it on my kayak without soaking the phone from paddle spray and the like.
I made a short video on the application I've been using:
The OtterBox phone cases are NOT waterproof. If you shop around, there are, however, quite a few waterproof bags that the iPhone, installed in the OtterBox case, will fit in.
The OtterBox 3000-01 is a dry box that might fit the bill. It's listed as clear, and waterproof to 100'.
Good point f_klock. The previous link I posted came as some interest to me being I think it may still give you the option of still being able to utilize the phones touch screen/access phone functions.
I like OtterBoxes. I have a few for kayaking though none of which are clear.