This video shows an easy way to make a outboard motor extension for your cordless drill.
The shaft is a wired rod of 10 mm in an aluminium square tube of 6 by 4cm by 90cm ending in a square drillhead. In the drillhead the 3 blade propeller of 24cm diameter. This part you can buy in a nautic shop for about 15 euro. All the other parts you can buy in the diy shop around the corner.
The left/right/tilt hinge is made from the same alu-tube and some alu strip. What you don't see is a long steering handle. The aluminium around the drill is a way to cool the machine. Don't forget to make some holes for the ventilation and the back/forward switch. The square drillhead has 2 holes on the side; easy for the connection to the alu tube. The total costs are about 40 euro / 45 dollar or 5200 jpy.