Matt Novak of Gizmodo introduces us to this marvel of automotive design. In 1921, John A. Cowan patented a car roof that slips off and turns into a lightweight flat-bottomed boat. The oars fit inside, so the user is ready to go as soon as the boat is in the water.
There's a risk of damaging the interior of the car if it starts raining. But perhaps that could be solved with a canvas cover over the open roof.