Google Maps also allows you to download locations for use without cell service or internet. Once you've downloaded the maps, all that's required is your phone's GPS signal, which typically doesn't involve paying for cell service. We had service last month, but we downloaded a map of the whole island just in case.

One thing to remember is that alternate routes based on traffic or road blocks may not be suggested if you're just using the GPS-only feature with downloaded maps rather than the internet. Your phone won't be getting the real-time data but will instead rely on what you've already saved. You could always check routes when you're connected to WiFi, though.