The "UDI" from UTC-Digital is a dive computer that includes remote S.O.S. transmission and SMS-like messaging ability. You can't actually input unique messages when you're underwater, but can instead choose from 14 pre-configured text messages, such as "I need help. Slow down." or "I chummed your weight belt."
The UDI units can communicate with each other or a base station on a boat from up to 500 yards away, which is pretty fantastic range for a radio system to work underwater. If a diver presses the "homing" button on his unit, the UDI can steer him back toward the boat.
All in all it's a neat system, which is probably why each unit costs around $1,500 a pop.