Heres a story my friend told me happened to his cousins:
They drift down the river in small boat with a 5 hp motor, then decide to turn around...and realize that the motor wont pull them against the current. It just stays in the same spot, but by paddling at the same time as going full throttle they can inch thier way upstream, that is untill they run out of gas. They ended up having to drift to shore and one them had to walk several miles to the closest gas station, and then they still had to struggle against the current to get back to the boat ramp.
This guy I worked with had a pretty nice 18ft boat, and he ran out of gas in the middle of the lake and only had 1 paddle.