Waterproof
Cell-Phone Cases
The best idea is stop into one of our sponsors and grab the appropriate case for your phone.
Here's the Dry Pak
phone case I use:

You can buy
it online. Google search: dry pak waterproof case and you'll be able to select a vendor for the case you desire.
I've
also heard good things about the AquaPac
brand of cases and used them in the past:

I almost always take my phone sailing. I've been able to call
home to let Sue know I'm running late. If broken and I can't get
home I've got a way to call for help. Fortunately after nearly 30
years of sailing I've always been able to get back in. But, just
in case, with the phone I have the option to call.
When you put the phone in the bag, whatever bag, give it a hard squeeze
to make sure it's water tight. I carry the phone around my neck
under my lycra or wetsuit and I don't even notice I've got it.
Avoid having anything sharp near the bag as it could poke a hole
in it.