How fast can you flush a toilet?
The boat in the top picture with the blown big block would go about a mile on a gallon of gas, tops. Half that if going over 30 mpg.
The performance of a jet boat suffers and the mpg is usually horrible.
Why? because people don't match the pump to the motor and versa visa.
You can bolt up a 500 hp motor to a 200 hp pump and all you do is waste gas. Too much pressure, not enough volume, thrust is not maximized.
They are not true centrifugal or axial flow pumps, kind of a hybrid mix of the two. But they do have pump performance curves and best efficiency points. An RPM where the pump moves the most water for the given power input and impeller diameter and discharge nozzle diameter.
Rev it higher and the efficiency falls way off.
if you can get a true pro to engineer the engine/pump combination, it will be much better.
it'll still cost 3 times as much in fuel and an outdrive or v drive would and will go half as fast, but that's better than 4 times as much fuel.
The Berkeley pump is best IMO, with a droop snoot.