[blantonl]

Flightradar24 works on the premise of volunteers providing ground stations to decode ADS-B transmissions from the aircraft.

Two variables here: 1) ADS-B Transmissions are line of sight, so once you are far enough off shore there are no ground stations to receive the data 2) Ground stations are provided by volunteers, so there isn't a critical mass of volunteers that provides 100% coverage over even ground locations