I don't really understand how a tunnel hull helps but I've been told that it does.
The tunnel creates a high pressure water zone directly in front of the foot of the outboard. In other words, it force feeds water to the prop, or in this case, the jet drive. It also maintains a water supply for the drive when there's not much water under the boat. They work very well, for shallow water applications.
With a correctly designed tunnel, you can actually raise the motor ( with a jack plate) and still provide water to the drive. You just have to keep in mind that if you raise the motor too far, the water pickup for cooling the motor can be out of the water. You will overheat the motor. Also, if you raise it too high, you can't steer. Ask me how I know that.