I use my cell for Navionics on the Q all the time. It doesnt require cell service to work, just GPS. Think of it like a GPS in a car, doesnt require any data/cell signal. Just a Satellite.
You can actually use an old phone for this as well. I have one i use for my yak. You dont even need to have it activated by your cell phone provider. Just connect to you home wifi, download the app and the map area and you are good to go. Bring it in the house and connect to wifi every once in a while to get the updates.