We have Verizon. We had pretty good 3G connections reasonably close-in to land from western Penobscot Bay on "south". However, in a number of places it wasn't 3G -- even close to mainland. Cape Cod, the south side of Buzzards Bay, even 15 miles off the coast in Mass above Rockport, are places that we got spotty cell service.

If your Navionics APP needs to have that connection (either because it's a cloud-like app, or whatever), then you'll not have the benefit of the service in number of conditions.

Does your GPS on the iPhone work when you have NO cell service?

I thought that GPS service with most cell phones was from a method of triangulating from multiple cell towers. Maybe it's changed? I would be a bit concerned about such accuracy. There was a little blurb on TV about how law enforcement has to use a special device to actually "locate" a cell phone within a couple of block area by on-the-ground, local triangulation.

Have you checked the differences in "marginal" areas offshore..