I agree the paddle wheel is a good, simple idea. But I don't think the paddle wheel by itself can tell you the speed of the boat. If I remember my dynamics class, the paddle wheel will tell you the flow in the wheel. Which will be a function of two forces; (1) thrust of the sub, and (2) thrust of the water. In other words if the sub is travelling against a strong stream, the paddle wheel (I think) will incorrectly state the sub travelling at higher speed. If the sub is travelling in the same direction of the stream, the paddle wheel (again I think) will incorrectly state the sub travelling at slower slower speed. If the sub is travelling in still water, the paddle wheel will correctly indicate the speed. I believe you also would need to know the thrust of the sub (i.e. propeller movement) to properly calculate the actual speed of the sub (or maybe I am all screwey - which happens quite often).