Ideally the centrifugal pumps will move water at a high enough velocity and force to act as a hydrojet, giving the ROV its motion. A centrifugal pump is quite a simple mechanism. Using an impeller, a vaccum is created so fluid can be drawn into the pump, when the fluid hits the impeller it is pushed out radially. as the impeller spins the water is released through another opening, and depending on the design of the case, the exit velocity can be manipulated. Very early stage designs:
Impeller design found on thingiverse.com
I'm not too good with blender, so cut me some slack!