FlightAware is coalescing a range of data. In the US they can use a time-delayed FAA radar feeds, feeds from ADS-B receivers, possibly other aircraft originated data (ADS-A/C), and fore-knowledge of the expected location based on filed plans and aircraft speed to date. In Australia there's little radar coverage over the interior or outer sea approaches: ground-based ADS systems are useful but there are significant blind spots and ATC relies on aircraft reports over voice (VHF, HF, sometimes satellite) and projections based on filed airspeed etc.
Commercial ships can routinely report a GPS location once an hour and it would be useful if the ship suddenly disappeared. They can do that by satellite or HF radio although I don't know the specific system. Ships rarely disintegrate in too short a time to issue a mayday or deploy an ELT. In any case they will be within, say, 20 nautical miles of its last reported location.
Emergency Locator Transmitters are the aviation device. They use a 406Mhz digital signal uplink to a satellite: the data carries a unique ID registered to the device/aircraft. Most emit a local homing signal on 121 and 243 Mhz that's not useful in a global sense. All have to survive the initial catastrophic event, and subsequent fall to Earth, to be useful. Some are automatically activated/ejected from the the aircraft after severe jolt, some are not. Some are activated by immersion, some are not. All should float if free of other wreckage. None work if submerged (either to receive GPS to get a location, or to transmit it). While certainly useful they are not an infallible system.