Am I understanding it correctly? This is a GPS that somehow (???) broadcasts your location to marine radios within an 8 mile range.
I'm assuming it displays the coordinates on marine radios ... somehow? Is that a standard thing for marine radios?
Digital Selective Calling radios are the new standard for VHF marine radios. It is complicated, but, yes, essentially they serve as an area 911 and will display or announce the GPS location transmitted and will have an alarm that sounds when an emergency alert is issued over the channel. GMDSS/DHC radios are required on commercial vessels.
Is 8 miles enough when out on open waters?
It would seem that a PLB would be a much better safer choice to protect if things go terribly wrong.
Do they make PLBs specifically for diving? I could see putting a regular PLB into an Otterbox or similar, but I'd wonder if the flotation factor would be too great. I'd hate to dive with a little box constantly pulling up on me.
The limit is a function of line of sight. At water level to a typical recreational boat it's going to be somewhere in the neighborhood of 4 miles, roughly. It will be somewhat further in a call to a commercial vessel with an antenna a significant height above the water.
This unit is deigned to deal with the situation that arises all too often where a diver is separated from their boat. Getting found can be a challenge, even with various non-radio signaling devices. Numerous instances where divers have been left, for a variety of reasons. It does happen. A PLB will get you rescued, but that isn't a local solution. Ideally, you would carry both.
Dive canisters are available for some PLBs. McMurdo makes one and ACR is planning to offer one with their latest PLB. Technically, an Otter Box is rated for 100 ft, as I recall, That may suffice for typical recreational diving, but I am not sure I'd bet my life on that. Typical dive canisters are rated for much deeper. The McMurdo is rated to 450 ft. giving plenty of safety margin.