Yeah it would work in theory; however, I imagine the range is going to be quite limited. The reason is that the acoustic receiver (transducer) is small...it has to be, it's on your wrist. My guess mirrors that of KWS in that I would expect a maximum range of about 1000ft.
Also as mentioned, you do not need a GPS underwater. You only need to get the signal from the GPS on the surface to the diver, which is what this product is doing. The only alternative would be to have a line from the diver to the GPS.
NOAA divers have used, in the past, a towed GPS. They zip-tie a GPS to a buoy and then drag the buoy above them using some rope while taking pictures of the seafloor. They can then geo-reference those pictures. It's a great idea; however, sometimes the error of the GPS can be quite high. Also, they have to take into account that the GPS might not be exactly above them...
My guess is that it is going to be priced somewhere around $1,000 because it's a two unit system (one on the boat and one with the diver). I could be wrong, but that's just a guess; anyone get a price from these guys?