DSC and AIS definitely beat a whistle, I’m a firm believer and these links will explain why!
My 100 minutes lost in the most inhospitable seas on earth
http://www.mcmurdogroup.com/wp-content/uploads/Clipper-AIS-MOB-Rescue-Case-Study-2014.pdf
This is really the same technology used in the Nautilus, with a VHF Tx at sea level and a receiving station. The thing about range is that it can be better than you think, it’s a function of the receiver height too. I’ve received AIS signals between sailing yachts around 100nm apart.
From a GPS function point of view if your system has a battery that keeps its memory powered its worth getting it going before you go diving, this is because each GPS satellite transmits almanac data that is used to align the receiver. If the memory is cleared it can take a long time to align at first as this data is received from the constellation.
This is the problem when you try to adapt a Man Overboard (MOB) protocol and technology from blue water open ocean yacht racing to the unique scenario of an isolated drifting Diver Missing/Lost-at-Sea:
Where divers get confused in the application, and assume "instant" local alert and rescue capabilities of an AIS/DSC VHF radio unit (i.g. current Nautilus Products like their Marine Rescue GPS), over a 406 MHz PLB is in the classic MOB (Man Overboard) Scenario:
Tactically this is fine for example, if you're crewing a sailing vessel in the biennial Transpacific Yacht Race from San Pedro Calif to Honolulu Hawaii, and you fall MOB during rough seas at night: Along with your crew, there will be anywhere from twenty to forty or so sailing yachts out there with AIS/DSC within VHF range in the otherwise trackless open Pacific Ocean ready to immediately assist in picking you up.
AIS/DSC VHF MOB devices are not meant to replace but rather augment PLBs. That said, since they alert nearby AIS-equipped vessels, such as the boat from which the person fell overboard, to assist in the search-and-rescue effort, these devices can reduce rescue times substantially. A PLB, on the other hand works worldwide -well beyond VHF range- to summon rescuers. It’s important to keep a PLB around if venturing far out to sea and away from other boaters.
A Lost-at-Sea/Missing Diver can be considered a special case of a "delayed" MOB Scenario: Delayed in the sense that an elapsed normal recreational depth dive time is about 50 minutes to an hour -->if the Diver is caught in a stiff current at depth, surfaces after an hour and does not see any sign of the diveboat, then the Diver has to manually call or activate a VHF DSC Distress Beacon such as the Nautilus Lifeline first generation or current Marine Rescue GPS,
but the caveat still being the limited range of the VHF signal along with hoping there are other boats or land stations in the vicinity with VHF transceiver reception capability. Again, if you're at a remote divesite hundreds of kilometers away from the commercial shipping lanes or land VHF receiving stations, then the direct to satellite PLB distress beacon is your last resort.
Again IMO/IME, a tropical squall & thunderstorm in zero visibility is the most common post-dive surfacing condition with a potential adrift lost-at-sea worst case scenario which you should smartly prepare yourself beforehand with at least a PLB:
A Nautilus Marine Rescue GPS/VHF Radio Transceiver Beacon may not save you in this instance due to "rain fade" signal attenuation, and the chances that any boats with marine VHF Radio DSC/AIS capability may be out of reception range of your distress signal by the time the squall clears and the weather, current & sea state moderates again.
Finally, a PLB (or even an AIS/DSC VHF Man Overboard transceiver in the links provided by
@Firebar above) will not save you over an hour's time immersion from potentially succumbing to hypothermia in very cold to temperate waters; but in contrast, the last chance and best odds of being rescued in warm tropical waters (
and from an agonizingly painful slow death from dehydration over several days) is always with a PLB. . .