Why is a propeller at the front of a plain but at the back of a boat?

Mainly ease of manufacture and operation. Airplanes with propellors in the back are generally more efficient in the overall drag and lift of the airplane. However, it is much easier to place the propeller in the front and the engine in the front as that helps with the Center of Gravity. Also placing a shroud around the propellor, just outside the diameter of the tip, will also increase the effeciency of the propellor. The more effiecient it is, the less power is required and the less fuel is needed or the engine can be made smaller. A propellor in the front of the boat would disturb or disrupt the water flowing around the bow. This would make the boat bump more over the waves it would create. Also a propellor in the back of the boat provides some protection in case the boat should hit something floating in the water.