I have a Pacific 22 that is fitted with a Hamilton HJ273 jetdrive. As others have pointed out, for manouvering at low speeds, I keep the revs at around 1200 RPM and then put the bucket down approximately half way, which effectively diverts half the thrust downwards and check my speed by raising or lowering the bucket. I can literally inch up very gently to an obstacle with virtually no speed, yet the engine is revving a little.
Keeping it in a straight line is quite an art and requires practice. It is easily influenced by waves and windage. I have passive hydraulics on the steering, which doesn't help either. There is a certain amount of slack in the system and the wheel never points in the same direction twice, so you can't judge where the nozzle is pointing. I am thinking of converting it to cable, so the system is more positive and the wheel will tell me where the jet is pointing.
Have been considering a couple of skegs to improve the straight line accuracy.
I have a couple of large Bennets hydraulic trim tabs that help a bit, by getting it on the plane quicker which helps with the straight line.
They are tricky, but rewarding when you get the hang of it.