Underwater Text-Messager

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 ("Watch out for that shark!") Over the same range, the devices can also be used as homing beacons so divers can find their boat.

There is also an SOS beacon which functions at up to 1000 meters (almost 3,300 feet) letting other divers and the boat know you're in trouble. The SOS beacon can also be activated remotely, so if your buddy looks like he's in trouble, you can activate his SOS beacon for him. And with an estimated eight-hour battery life (assuming one message being sent every four minutes), this thing won't die right in your moment of need.