UTC (Underwater Technology Center) has developed the Underwater Digital Device or UDI, which allows you to send text messages underwater.
The UDI is a wrist-wearable diving computer that not only keeps logs of your dive, but also allows you to communicate with other divers and your boat. The UDI uses 2-way text-messaging, over a distance of up to 500 meters (or about 1,640 feet). There is no keyboard, so divers send any of 14 preset messages. Over the same range, the devices can also be used as homing beacons so divers can find their boat.
For further information see this link.
