After going out on a limb with the last question, I thought I'd add another that suggests using air compressing technology on non-compressing water.
Would it be possible to combine the use of a movable inlet spike like on the Lockheed SR-71 to change the shape and size of the inlet pathway?
I'm thinking I could combine automotive turbocharger/supercharger technology with jet engine flow technology and end up with a efficient jet boat.
There is probably a good reason why this wouldn't work (ie, water is not air), I'm just thinking outloud.
Back to the drawing board.