place your bets now for when the n20 kits become available that feature water/metanol as fuel instead of gasoline, and feature a stand alone H2o/methanol tank. its coming.... i assure you!!
the reason N2O needs extra fuel, is the oxygen molecule needs something to burn, instead of melting your piston like you applied a oxy torch to it. Adding water adds even more oxygen molecules (H20). also, the nitrogen molecule in nitrous has a cooling effect, which is what water can supply as well.
Just because you can't do it...doesn't mean its impossible
what does the methanol and hydrogen in the water do? i know that N2-0 has 2 nitrogen molecules that are inert and are not consumed during combustion. H2-o has 1 oxy molecule, just like N2-o does.