just be sure you're not going out of cell phone tower range .... as there are a lot of cell phone 'holes' (no reception) in the Carolina and GA 'low country' ... ditto for well offshore, too!
Not correct IMO. I think you have stated a common misconception about cell phone GPS functionality.
Except for a very few restrictive plans that block GPS on low-cost "feature phones," you do not need cell phone reception for the GPS to work in a smartphone. I have never owned or heard of a GPS capable Android phone/tablet, iPhone, or iPad that did not work when away from cell reception. They might get a slightly faster initial fix with the added benefit of cell tower triangulation, but once you have the first fix you'll get a fast fix after that since sailboats don't go very fast. The satellite reception is all you need, and that's available basically everywhere except around skyscrapers and dense forest.
My Nexus4 had no cell service in BVI - in fact, I kept the radio turned off to avoid accidental roaming charges. Yet my GPS worked perfectly. Ditto for my son's iPhone.
My iPad has AT&T wireless capability, but we've never purchased the wireless service. (My son bought us the wireless model to get the internal GPS, even though he knew we would not subscribe to the wireless service.) That iPad also worked flawlessly in BVI (using Garmin Bluechart app).
If you know of a specific model of phone whose GPS does not work without cell tower reception, please let us know what model it is so sailors can avoid it. But until I hear about those models, I will stick with my own observation that everything I've tried works fine. And I would bet that any GPS capable phones that don't work could be rooted to work.