What am I missing? The GPS capability seems all but wholly superfluous, unless you have a bathymetric chart with adequate resolution to be of interest to a diver ranging over a couple hundred yards. That can be seen on a what, 30 x 30 pixel wrist screen. That have any bathymetric relief or features prominent enough that you can orient to them. I can't recall seeing even surface topographic charts with that kind of scale.
Not that an acoustic location system with good directional precision and range estimation wouldn't be a great thing to have. I've long wanted one. Just don't see the point of GPS...
ETA: Ah, waypoints are a nice feature, I guess that's where GPS comes in. You can find that great lobster hole or wreck again next week. That will add a new dimension to murky NorCal diving - a second dive to the same spot!
Even if the precision of the locations determined acoustically aren't as good as claimed, it would be a nice thing to be able to map the relative positions of the divers around you. You could easily regroup even in somewhat bad viz, like we do topside when we're roaming about.
I dont think the gps part of it is for what you are trying to use it for. rather it is a foundation platform that its functions are calculated from/on. your exact position i believe is not that important as the relative position's as you find valuable. some place in the process is using the gps to determine the relative positions from a known fixed point and relaying that data. then the wrist units is probably showing the lat/lon calcs on a scalable display.
ie i am 2 blocks norrth of the firestation and you are 3 blocks south. we are 5 blocks from each other. the 5 blocks are relative from a known posit.