I agree with a previous poster and go to a sporting goods store and get a waterproof case. I always place my phone in one of these before I go fishing, and even if I get drenched, the phone stays well protected. Less than $15 bucks and protects better than an Otterbox. In fact, I think Otterbox makes a case like this as well.
If you don't go this route and want to go the cheap route and use a ziplock bag, make sure it's a freezer bag and not a thin one. If you wanna be extra safe, put you phone in a ziplock then put it in the case, that way when you use the phone and take it out of the case and accidental drop it in the sand, it's still protected. Better safe than sorry. From personal experience, stay away from those "waterproof" ziplock style cases that look like really thick ziplocks. Those do leak.