On my last trip to Cozumel, while waiting on the surface for our boat to see his smb my DM mentioned the piece gear he would like to have most would be a handheld VHF to call the boat. He said there are so many boats in the area that even if he could not raise his boat for some reason someone would hear and either signal his boat or come running if there was an emergency. I would think that like a PLB one could find a case to fit a handheld VHF and in many dive sites it would be a extremely useful dive aid with a much quicker response than activating an epirb.
This is a great idea for someone who is very familiar with their diving locale and could accurately describe to another educated listener.
But for the sake of argument, without telling me any information other than what you can see in 360 degrees, tell me where you are and I'll guess. I'll try too. I'm in a hotel room, outside my window I can see a canal. Across the canal there's a building that says Hitachi on it. There's also a tower looking building that has a giant O on the side of it. There's a highway and a beach on the other side of the highway. It runs perpendicular to the canal.
Anyway, hyperbole aside, without being able to accurately convey your location, a radio is only good enough to serve as a notification, and only if anyone is listening. The range is much shorter as a floating diver compared to even standing on land, and you are dependent on your emergency call activating enough resources to find you, without always being able to give a reasonable location. "I'm a mile offshore. I see palm trees. I see sand." Since most small boats are not equipped with direction finding equipment, they would be hard pressed to narrow down your location. A transmission area of 1 mile in any given direction means at the very least the boat could head the wrong way. So now you're two miles away, no further radio contact is possible, and drifting. Add some time for them to return to where they first heard your signal, and now you can't reach each other at all.
Please don't take this as dismissing your idea. I think in a great many scenarios having a functional radio would be an awesome idea and very useful. However it shouldn't ever be considered a replacement for a PLB, and if you're in a place where most marine radios are AIS capable, it's not a replacement for a PAB. In a place like Cozumel, it's a great idea, the chances that there is someone who is knowledgable and nearby is much greater. When someone like a DM has one and can talk to a skipper and both know the area it is most certainly going to be faster than a PLB or PAB. For someone who is unfamiliar with the area, or speaking to an unfamiliar listener, the utility is greatly diminished.
As a supplement to a PLB, PAB, and signal kit, it certainly has merit in certain circumstances. However it's not a replacement except in a very narrow set of conditions.