The thought just came across my mind and I simply cannot think of a good answer, especially not knowing much about the mechanics of it all.
In: Engineering
In both cases the propeller is actually more efficient when mounted on the rear. This is because the inlet cone on a propeller is much wider then the exaust cone so the hull of the craft is not so much in the way. However the problem that aircraft face is the issue of weight distribution. For stability reasons you need the weight of a craft in the front of the drag. The best example of this is an arrow or a dart which is very stable because it have a heavy point in one end and a tail for stability in the back. Airplanes are designed with the same principle. But in order to do this they need the heavy engines far in the front. Aircraft that use a pusher propeller therefore have to run a long prop shaft through the entire length of the tail of the aircraft. Some boats does have a very similar configuration but because they do not have to fly they usually have much smaller engines and propellers compared to the size of their ship. So they can easily get away with running a small propeller shaft along the keel of the ship to a center mounted engine.
There is also another reason which is why even aircraft with wing mounted engines usually have the propellers in front of the wings. It has to do with low speed control. A big issue with low speed flying is that you do not get enough airflow over the control surfaces to be able to control the aircraft. One solution is to put the control surfaces in the blast of the propellers which gives them more airspeed. And because there are control surfaces on the wings you therefore need to put the propellers in front of the wings. Ships do the same thing but with a slightly different configuration. They put the rudder behind the propeller but the rest of the ship is still in front of it. If you look at some more extreme aircraft designs you do sometimes find this with pusher propellers as well. But cutting out parts of the wing or hull of the aircraft in order to place the propeller in front of the airfoils but behind the rest of the structure is often counter productive.
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