Allright, here it comes.
Would be more appropriate in the public safety diving forum, but I guess it's exciting to watch for all.
One just needs to place three sonar buoys in a triangle and then take this tablet on the dive. Accurate position in real time on a map. Route monitored and recorded on a lapton on the surface, too. One needs to be aware of sonar reflections from hard vertical walls though, but that's doable. Artifacts can easily be detected (your position jumps 50m in a second and you'll know there was a bad echo). Complete the system with a phoneline to the diver and great things can be achieved even without visibility. And oh, I have no financial interest in this system (but I did get free coffee and a really nice experience).