There is no need for cell service to use GPS on a phone. Get a good case, otter box is great for protection, I drove over my iPhone on a cement driveway and not a scratch with the otter box. Life case is waterproof.
Navionics for a phone is a decent app but I don’t think it is quite as good as a GPS unit. I went with the Navionics app and sold my Lowrance H2O and chips. The app was $10 and I got $250 for the H2O, 2 chips and the power cord.