ArcticDiver:Mike, the idea of a Buddy Finder isn't that attractive to me. My experience is that most divers, not all, will get split from their "buddy" no matter the visibility or circumstances. Adding another piece of technology isn't going to help.
But, if you produced a reasonably priced Beacon that would interest me. Then in low viz, instead of keeping my eyes on my compass I could just place the Beacon; occasionally verify my heading back to it and go have fun. Or, when I find that one really interesting place among all the similar looking coral I could mark it, go find other folks and then come back smartly.
Emphasis here on reasonably priced. It should cost no more than a Nano, or similar.
Here it is. http://www.desertstar.com/newsite/sport/sportscout.html You can buy it at http://www.leisurepro.com/Prod/DTSC.html its a good to find your way back to some thing. Maybe two divers and two beacons, that way you could find your buddy and they could find you. I dive with Mike sometimes and some of the viz is very low. You can spend a lot of time going back up surface to find your buddy. With Mike's idea you just go to the becon he's carrying or your buddy can return the beacon you are carrying.