When I had a boat, I had a Digital Antenna Systems repeater (repeats the signal).
PowerMax 4KMR-30M 50dB Gain Wireless Cellular Repeater System - 3,500 Sq Ft Cell Booster
It had a high-gain 6ft antenna sitting on top of the radar arch, so it was 20 some ft in the air. And when you consider we were on water, then we had the best conditions we could have.
When the system worked, I could get 10 miles out and still have a cell signal. But it was really finicky, and not reliable enough for use.
Of course, the VHF, Radar, and other things on board that transmitted signals might have had something to do with the flakey operation of the system, but if I were going to try one again, it would be one of the Digital Antenna System units.