The jet unit is basically a water pump.
Water is drawn in through a large intake then forced out through a smaller opening under speed and pressure. you are forcing out the same volume of water from the large intake through a smaller outlet.
Before this leaves the outlet it has to go through a stator, this concentrates the water in a straight line, without the stator the water would just spin in the same direction as the impellor that is driving it.
The jet can be used to great effect on all kinds of vessel from heavy displacement to fast planing craft.
the majority of fast cat ferry's use water jets, the vomit comits use four water jets with 38,000 HP driving them.
The infitely adjustable directional thrust gives excellent maneouvarability(think thats spelt right ad a couple of wines) and towing power over a propellor.
You will not get the same top end speed as a prop, but not far off.
If a jet leaves the water a lot of people say it takes a long time to pick up again, not true, takes no longer than a prop.
The other advantages are, no exsposed propellors to worry about, less mechanical problems as most jets have only one moving part.
Can explain more on Sunday Kathleen
and would welcome any sceptics to run out should they wish