DGR thanks but you're mostly telling me what I know.
JF, you're at where I was a couple of days ago.
It's not a bow thruster cos that's just a propeller in a tube. However, waking up with a tad more brain power than I had last night has been a help and I think I'm getting closer.
This morning's puzzles. I was considering why there is a recoil when a bullet moves down a barrel but there would not be one if it got stuck and the pressure was simply turned to heat.
The bullet is absorbing energy because it is free to move and it is storing this as kinetic energy.
The energy will be released when the bullet hits something but that is only minorly significant cos that's where the waterjet is going to get it's manoeuvrability from.
My conclusion from this was that there needs to be something moving and in the waterjet is has to be water.
The waterjet needs to be able to hold water at a high pressure to provide the propulsive force and so it cannot simply be an open tube. Also we need to get the moving water (the bullet) to store as much energy as possible. The stored energy is going to be largely wasted out the back. Kinda like the bullet after it's left the barrel.
The way to get the energy into the water is to increase its velocity. Also, the more mass of water we can increase the velocity of, the better because it will store more energy. However, there will be a limit to the mass of water that can be got moving because if the hole out of which the water is flowing gets too big, the pressure inside the jet will begin to fall and it's this pressure which will make the boat move forward.
Enter, the nozzle. The nozzle and the taper before it is the device which is going to accelerate the water to increase its velocity and, hence, absorb energy at this end of the system. This is the bullet. So, now we've got something for the pressure at the forward end of the jet to react against. Of course, because the water jet is capable of drawing in water, it is able to constantly replenish the bullet. In turn, the boat will absorb the energy imparted to it and also store some of it.