Cave Diving for Dummies
Even expert cave divers occasionally drown.
Not too long ago I read a story about a couple of cave divers who were videotaping their adventure. It was clear from the clock on the tape that they were already dead and didn't know it (too far in to make it out in time). That's where the robot retriever comes in. Divers would set a timer on it and leave it in the water behind them. The timer would be triggered when they were half through their air supply. At that time it would go after them, bringing a fresh tank. Finding them might be a little tricky. Don't know how well GPS would work underwater but a trail of RFID tags might do the trick. Not only would the tags let the remote rescue device find them they could also be used to find the way out (this trail would be useful even if there was no rescue device on the way - disorientation is a big danger when cave diving - the tags would be able to show them the way out).
The price for the robot might not be as high as you'd probably expect either - it would really just be a modified version of the Roomba that people buy to vacuum their houses now. There'd have to be some redesigning of course (for one thing enough buoyancy would have to be added to make carrying the extra weight of a spare tank less of an issue) but it seems doable.
Another couple of points to consider:
1. The electronic trail marking is not only for the robot. A lot of time sediment stirred up by the divers reduces visibility to zero. An electronic path back to
the cave entrance would take care of that.
2. Many times it's inexperienced divers who don't pay attention to anything except their own clowning around that die. Borrowing a suggestion from a user at Half Bakery who goes by the name of Texticle and modifying it a little, a puppet show suddenly appearing superimposed on the inside surface of their masks might get their attention when their tanks were half empty, especially if the puppets were suffering a gruesome death by dying. The puppets could even have names under them like avatars - only in this case it would be the names of the divers.