Buy a proper ziploc bag. Not a "dollar store" brand, a real Ziploc brand sandwich bag. The one with the double zippers. Then just keep your phone in your pocket. You can still take a pic (the plastic will still let you use your touchscreen) and your phone will stay bone dry. Even if it happens to fall into the water. I always use one when I go anyplace that it might get wet.
Just squeeze out all the air before you seal it up tight so that it's easier to handle.
Seriously. Best "completely waterproof case" I have ever bought. And you get 100 of them for less than 5 bucks.
ps - my everyday case is an Otterbox defender for the bumps and knocks.