I'm no expert put I'll toss in my .02.
On my jet ski's, when I first hit the throttle the stern does squat for a second, especially is shallow waters. I know that anything that isn't nailed down when I pass over it will get sucked up into the pump. In some circles it's called "The Hoover Effect". Like the vacum cleaner? So, I'd say it's possible for the pumps to suck the stern down. Those big pumps have to suck a lot of water before it make thrust.
I have read in magazines that a hull has to be specially designed for water jet application. I understand that you can't just install a jet pump in place of shafts and props. Something about the hull design and how water flow is directed into the pump intake.
Like I sad, my thoughts and observations.