A diver's mask with a special display lets the diver see his or her real submarine surroundings overlaid with computer-generated virtual scenes. In the pilot application, an AR game, the player sees a coral reef with shoals, mussels and weeds, instead of a plain indoor pool. Applications for professional divers are being investigated.
However, the implementation is a little bit lacking. The divers need to carry an ultra mobile computer on their back, and visible markers are used as reference points for the AR algorithm. In the future, Fraunhofer are planning to adapt this setup to aid in underwater maintenance tasks.
Fraunhofer press release is here, where you can read further details about the technology and the game using it.