Let's suppose, contrary to popular belief here, that the water powered engine is viable. You still have to build it, and get the batteries (and how do ya charge the batteries) etc... If the raw parts are roughly equivalent to an ordinary engine, then cool, it's on, but if it's way more expensive, when do you catch up?

It's a bit like the petrol/diesel argument, in our country anyway. Diesel doesn't get taxed anywhere near as much, so it's cheaper, except that diesel vehicles have to pay seperate road-user charges, and the engines cost more, so where's the pay off? Ditto the hybrids (at the moment anyway), you end up having to drive a lot of kilometres to justify the added cost, kilometres you'd perhaps not normally drive, but, having bought one, you feel you should.