While reading about the underwater GPS. The units that are attached to the vehicles are only receivers? which means that if my ROV knows about its position it is only through the tether communication. Which means if I operate without a tether, the underwater vehicles will know nothing about its position am I right?
Your understanding is correct, the master electronics box (at the surface) is the thing that calculates the vehicle position. Then the master electronics box sends the position information to the vehicle through the network (on the tether).
The underwater gps system alone cannot send the position information to the vehicle without a tether.
Using the Modem M64 you get a two way acoustic data link. It can be used to transmit the position from the top side to the underwater vehicle, but it will of course introduce latency in the position compared to using a tether.