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Please check out UWIS Systen in www.uwis.fi. Ready and really worksBasically, as GPS signal cannot go deeper than a few centimeters underwater, so GPS cannot be used. Underwater Navigation Systems use two types of technologies :
1) Inertial navigation
2) Acoustic signals
For 1) the system tracks accelerations and magnetic headings to define the current position referenced to the starting position.
For 2) the system tracks the direction from where the acoustic signal comes, so as you can navigate to the source of the signal.
The system 1) has only one device carried by the diver. It basically will have a screen where you can see your course and you can reach the starting point marked in the initial point of the course.
The system 2) needs to install a transmitter in the starting point (for example the dive boat), so as you are guided back to it.
Systems like 1 are still under development, are expensive, big and heavy.
Systems like 2 are available but you need to install the transmitter in the starting point and no one should touch it.