Hydrogen can actually be separated from water quite easily with no energy input. Just put a piece of aluminum in into some water and hydrogen will be release as the surface of the aluminum oxidizes. The problem is that such processes are not sustainable. Once the surface of the aluminum is fully oxidized, the reaction stops. After this, the material needs to be reprocessed so it can be used again. This part requires significant energy input. Genepax has not revealed details of their process, however it is believed to be something along these lines. While the converted Reva electric car may indeed be able to run on water for a short time with no external inputs other than water, it's not clear how long that can actually happen. It may turn out that Genepax has come out with something revolutionary. More likely they are only showing part of the big picture right now and we aren't seeing the other part of the process that regenerates the energy generator.
[Source: Reuters, thanks to David for the tip]