One of our local news stations in Hawaii did a story on a similar system. This guy has already installed 100 of them. Selling them for $495 installed. The kit alone is $359 which looks like a modified thermos.
Here is the link to the article which also has a video of the news broadcast.
Can Water Boost Gas Mileage? | KHON2.com - News, Sports, Weather | Local Top Stories
I did not get it, I would wait a while to see more results from others.