Wow... that's a really good idea! If you dive with a group, chipping in to have one would cut the cost too.
My Brother in Law, has a watch that is similar to this, but only transmits a locator beacon, on the 121.5 Mhz band. He had it for when he worked for the UN, going to all those wonderful places that the rest of us try to avoid. (ie the mass graves in Kosovo)
I'll have to ask Andy if his watch is dive proof, the cost, and where he got it.