I have been wanting to build a micro ROV for some time now. Wanting to keep the propulsion system simple I thought i would use some kind of pump to generate thrust rather than having to deal with sealed propeller shafts.
I have now discovered these cheap submergable pumps. They run on 12V, have an intake in the front, an outlet in the back and generate a very decent thrust when simply turned on. They can be PWM controlled to deliver a very precise thrust.
Flow 450 l/min.