Cellphones need to be reasonably close to a tower to connect to the network. Grady's boat is so far off shore that land is not visible, but he is able to receive a call and send a text on a normal cellphone.
In the last of the shot's as Harlan's boat comes alongside Grady's, the wake of the camera boat can be seen in front of both.
In the opening scene Bree wakes up in an unlocked state room on a yacht at sea with no restraints on her hands or ankles. No kidnapper would make that mistake.