Water is an excellent Ground Plane for signal travel, since it's "relatively flat" and free of large signal reflecting objects. Signal (Radio Frequency Transmission) can "skip" great distances over water and under the Ionosphere cover. I am a former CB Radio operator and have QSO cards from people I've talked to that are in icountries thousands of miles away from me here on the East Coast. Skip, or "DX" as it is also called is far more common in the longer wavelengths such as CB.
The frequencies that Cellular radios operate in are less likely to "skip" the great distances that longer wavelengths, but another phenomenon known as Tropospheric Ducting will cause typical line of sight transmissions (which would normally begin to dissipate near ground level as distances increase), to instead "bend" and follow the curve of the Earth, allowing even short wavelength signals (UHF, VHF), to travel as much as 500 miles (correction) in some cases, where normally they would continue in a trajectory line of sight and would have long since dissipated at ground level under normal circumstances.
Think of it this way, take a ball (as the Earth is round), and hold it up, and then look over it. The line of sight you are looking at from where it touches the top of the ball to where your eyes hit the wall across the room is the Line of Sight. Since you can't see past the top of the ball down around the back of the ball, likewise signal won't reach that part of the ball. Tropospheric Ducting would allow your eyesight to "see" the back of the ball beyond the top of the curve.