Can a flame start on water (video included)?
Watch this video from 1:50 onwards to see the reactions of sodium and potassium with water. https://www.youtube.com/watch?v=H6ZDiRIvc2E

This video, 0:56, shows  gasoline (petrol) burning on water. (Note that the batteries have nothing to do with it). https://www.youtube.com/watch?v=Bs13jBWqFjA