An aircraft's jet engine, by contrast, accelerates a smaller volume of air by a large amount.
In an internal drive boat, pumping a small volume of water and accelerating it by a large amount delivers the thrust.
Efficiency of the drive is related to the difference in speed of the vessel and the accelerated water producing the thrust.
Jet drives are inefficient in low speed vessels, but may have other advantages that make them suitable for a given application.
Internal drive propulsion was originally designed by Sir William Hamilton (who invented the 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.