You need to watch just upping the voltage, because the current that the motor will draw will rise massively as the prop rpm increases (it's a cube law relationship, so doubling the voltage, doubles the potential prop rpm but the power goes up by 8 times).
You might be able to get away with using a brushed ebike controller as a speed controller and voltage reducer, by limiting the throttle travel to restrict the maximum motor rpm. This would let you run the motor at normal power and act as a voltage to current transformer, with some current limiting protection built in. You'd draw less current from the battery pack, too, in proportion to the ratio of the voltage drop.
The cheap Minkota motors don't have PWM speed controllers, just a couple of resistors in the motor housing to switch speeds (they call them 'speed coils'). There's an interesting thread on the Boat Design forum where a chap has experimented with making one of these cheap Minkotas more efficient; he used a brushed motor controller. See here: http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-15.html#post302305 and also his final pictures, here: http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-28.html#post381261 for some ideas that might help.
Jeremy