A cellular system engineer
once told me that the maximum range is 32 miles, regardless of your equipment
or power. This is because the systems (all of them) use software
designed to look at the time lag in your signal, and if the tower itself 'sees' that you are 32 miles away, the system is designed to say "Well, there must be a close tower for him" and it will not connect you to the system. Period. That's done to prevent strong signals from going "chatterbox" and engaging too many towers and overloading the switching logic in the system.
For distances less than 32 miles, you still need adequate signal strength as the cellular system uses the "FM capture effect". That is, if a tower can handle 100 calls, it takes the 100 strongest signals. If you are the weakest and a local passing motorist comes on--you get dropped, they get picked up instead.
So if you are looking for offshore range, plan on being well inside 32 miles, and using a bidrectional amplifier, and a gain antenna
. With just the phone
in your hand, your range may be only 3 miles. It all depends on how your carrier has their towers set up, and how much competition there is to use them.
I've played "bar bar, who's got some bars?" and even 2-3 miles can be zero coverage if your carrier has their towers set up to handle the area where most users are--on shore, not off.