We do use an Ipad and it's GPS for charting at times. In an emergency a phone with a plotter app is ok but barely. A tablet with GPS is much better.

An option might be to get a less expensive new phone and with the money you save get a GPS tablet (even from Craigs List). With a GPS tablet's larger screen it's a lot more versatile as you can actually see it and you don't need a data plan.