When you rebuild the jet ski pump, you gain performance and improve the quality of the ride. Nicks and dings in the impeller impede performance and make for a rough ride. Worn our shaft bearings and excessive clearance in the housing between the wear ring and the impeller can cause your jet ski to loose power and speed. Also, a worn out drive shaft, bearing carrier and/or seal can result in a ski taking in water and sinking. As you can see, rebuilding your jet ski pump is a relatively inexpensive and easy way to regain some performance that can be lost over time.
What does the jet ski pump do? Basically, it is a function of differences in the mass-flow of water. Water enters and exits a jet drive at differing velocities, power is converted to thrust via an impeller, and the thrust propels the jet ski. Steering right and left is accomplished by directing the system's exit flow in one direction or the other, much like directing the thrust of a submerged propeller-driven system. Forward and reverse motion is infinitely controllable through a reverse deflector that simply diverts the thrust fore or aft. This type of system is common on newer skis.
Jetboats were originally designed by Sir William Hamilton (who developed a waterjet in 1954) for operation in the fast-flowing and shallow rivers of New Zealand, specifically to overcome the problem of propellers striking rocks in such waters. One very important feature of the jetboat is the fact that it has no external rotating parts; it is essentially safe for swimmers and marine life, excluding being struck by the hull. This in itself can be reason enough to use this type of propulsion.