How far offshore can a vessel reasonably expect terrestrial mobile coverage to remain usable?

There is no fixed offshore distance. Coverage depends on coastal tower height, frequency, terrain, antenna height on the vessel, weather, network configuration and whether another landmass provides service. Mobile networks are designed primarily for land coverage, so maps may not represent marine performance.

A good external antenna and elevated position can extend usable service in some areas, but it should not be treated as a safety or guaranteed offshore communication system. Test the intended route with the selected carrier and maintain appropriate marine communications for operations beyond reliable terrestrial coverage.