I lived and worked full time on a boat for many years. The answer to your question is in three parts:
1 - normal cell hand held phone use, you can often get (patchy) reception up to 15 (more sometimes) nautical miles off shore if lucky.
2 - setup a wifi hotspot on your phone and pull it up the mast. Range increases, i'm not sure how much to, but at 15nm the reception is good. Can work out further too, i've been on the far side of the channel islands in CA and had (poor) reception
3 - device with directional antenna up the mast - I always intended to do this but never did. I expect probably can get genuinely good coverage at 20nm out, maybe a bit more
1 - normal cell hand held phone use, you can often get (patchy) reception up to 15 (more sometimes) nautical miles off shore if lucky.
2 - setup a wifi hotspot on your phone and pull it up the mast. Range increases, i'm not sure how much to, but at 15nm the reception is good. Can work out further too, i've been on the far side of the channel islands in CA and had (poor) reception
3 - device with directional antenna up the mast - I always intended to do this but never did. I expect probably can get genuinely good coverage at 20nm out, maybe a bit more