I know some commercial fishermen in California and noticed they like to post on social media while at sea. I asked and it turns out they for the most part can get LTE coverage a good distance offshore, to the point that while dragging nets in water 10,000 ft deep they still can use their phone. The cell signals travel much further over water compared to over land.
I will ask one of them for an idea of how far they get when they lose coverage and edit when I hear back. He is on the boat now.
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
I will ask one of them for an idea of how far they get when they lose coverage and edit when I hear back. He is on the boat now.