I've just tried out a proper Kort nozzle on my thruster and it seems to work a treat, definitely more consistent thrust above 1kg, I did see over 2.3kg on the fish scale, equating to about 1.15kg from the thruster, but here's a shot of 2.22kg, or just over 1.1kg at the propeller.
Here are a couple of pictures of the new Kort nozzle mounted to the thruster.
It's a lot closer fit around the propeller than the shroud I made earlier, which is probably increasing the thrust.
Next step is to manufacture a mount for the thruster itself, so I can attach it to the frame of the ROV..... it's still only in "Martin's 3D World" at the moment, and unfortunately so is the frame of the ROV for that matter :)