You can check out the original concept and current production water-cars at WaterCar.

 

It was supposed to catch on, and then natural gas went way up in price, making it much more expensive than gasoline.

Car conversion kits that used to sell for hundreds couldn't be given away on ebay. You can convert any unleaded car to natural gas supposedly.

If you use natural gas, your car may not release many pollutants, but all you've done is effectively transfer the pollutants to another stage. In this case, it's the harvesting of natural gas that causes so much pollution. Same goes for electric cars. You can't say you're green when your source of electricity is probably a coal-burning power plant.

 

I heard that natural gas is more knock resistant then gasolin (better then 92-98 octan)

 

I can.. we are about to get our power from Oyster Creek Nuclear plant as soon as the oil burning plant known as B.L. England goes off line next year.

Another thing to remember about natural gas, I think that volume for volume, it holds less energy than gasoline, so you get few miles per gallon and less performance.