It is held by passive magnetic rings at either end. Stronger ring at the end where water is sucked in, because that's the way the prop will try to go when powered up. You'd have to push it pretty hard to get it out of place. But it might be safer for the system put the prop in a housing and connect tubes to inlet/outlets. The main goal is to pump water with little friction, so less powerful motor is needed. Less noise. The paddlewheel and piston pumps are too loud.
If you had neodynium mags in there it'd never come out of place
Put a 10 pound force mag it it.. Just be sure to shield it from pc components.