A few years back I found myself in a situation that no diver wants to be in. I had surfaced from a night dive with my buddy and our boat was no where in sight. We were adrift off of Grand Cayman and pretty far from shore. To top it off the current was taking us even further from the island. Our hopes were high when we heard a boat start up even though we couldn't see it. But it was short lived as we heard it the boat drive off into the distance.
After a nerve racking hour and a half adrift with no boats in sight we finally had a boat in view headed our way. It was our boat and our nightmare was over. They had apparently done a poor head count and returned to the hotel before they noticed that 2 tank spots were empty.
Today we just received a shipment of a new product at AquaSub Scuba that I wish I would have had with me. It's the Nautilus Lifeline. This compact VHF radio and GPS unit should be a must for every diver. You can radio to the boat, on the coastguard monitored station or send out a distress signal with GPS location. It's also waterproof to over 400ft so safe to take on all your dives.
Hopefully you'll never have to use it but you'll sure be happy you have if you do. I know it's going to be a part of my kit!