yeah what he said about the impeller, thats the water pump, so its almost maintenance, dump the out drive oil see if its milky, you'll know its leaking oil/ letting water inside it. most of the time its the seal behind the prop for the shaft.
they sell a great pump to with a hose, ( 5 to 10 bucks) it threads right into the out drive drain hole, and threads on to the top of the gear oil bottle, you put the hose in the drain hole, and pump till oil comes out the top hole, its a drain screw just like the lower one, when oil comes out, put the cap back in then take the hose off the bottom drain and cap it fast.
if you take the top plug out then the lower one, the oil will come out faster, its thick and slow. unless its a milk shake it will just pour out.