And, you don't even need to build a water-to-hydrogen generator for your car to test the theory. The only bits of information you must know are (1) exactly where to introduce the gas into the intake, and (2) exactly how much gas to meter in based on RPMs. Find out how much hydrogen flow is required at 2,500 RPM and go ahead and try it on the highway for 20 - 30 miles and see what happens.
If using a tank of compressed hydrogen yeilded the increase in fuel economy these people seem to indicate then it might pay to go ahead and build yourself a system.
My advance guess, however, is that all of this is complete and total bulls**t.