This is a pair of fairly high performance 24mm waterjets that I constructed for a boat that I was designing.
Mount for micro servo on top, I use TG9e, but any servo of similar dimensions will work.
Suitable pushrod is piano wire, less than 2mm thick.
Impeller axis are 3mm, and can be sourced from scrapped CD/DVD drives, if not bought from hardware store. The axis hole in the impeller and through the housing are made slightly too tight, and should be drilled to 3mm to ensure a tight fit.
For the coupling between the motor output shaft and the impeller axis, I used a silicone tube that slides over the joint and holds them by friction. Simple but works, and tolerates some misalignment between the motor/impeller shaft.
To lock the impeller on the axis, I took a connector strip (the type you can use for joining electrical cables) that had 3mm holes and carved out the metal part. That left me with a metal tube with a lock screw in each end, that i split in half and grinded down slightly to fit in the slot at the base of the impeller. (Picture can be provided upon request)
To make a watertight fit between the hull and the outlet, I bought a 1mm rubber mat, originally intended for gaskets in cars, and cut out a gasket that fit between the outside of the hull and the stator.
Underneath I painted the hull and let the paint fill seal between the hull and the inlet.
Test run: https://www.youtube.com/watch?v=2UF496_0Dro