SkimFisher
Contributor
I have seen a unit that flashes a color coded series of lights based on buddy distress, just need attention, or "hey look at that." I am pretty sure that it has a short range, but sounds like what you are talking about.
I am sorry that I do not know what it was called. I thought that I saw it in a Dive Training Magazine.
I usually do that (minus the head smack) and grab his/her fin.
If you want god communication underwater, you could always pick up this book.
Yes, I am an advocate of the "fin-tug" as well.