ROAB is a patented system composed of two units for finally being able to answer that question. One unit is on the mark. This unit sends out both a radio signal over the surface of the water and an acoustic signal below the surface of the water. That is why the system is called ROAB: radio over, acoustic below.
The second unit is on the boat and it receives both the radio and acoustic signals. Because of their relative speeds the radio signal essentially arrives at the boat instantaneously. By timing how long it takes the acoustic signal to arrive, the distance to the mark can then be determined.
Finally, a definitive way to know when a boat is inside or outside the zone!