no energy conversion system is 100 percent effective, let alone more than 100 percent.
so.... you take water, break it into hydrogen and oxygen, then burn it creating water again......... No mater what, you are less than 100 percent efficient. couple that with the efficiency of a internal combustion engine of around 25-35 percent, you don't come out ahead.
and water is not lightweight to carry around. to get much energy you need lots of water.
better to use regnerative breaking to charge a battery, with 90+ percent efficiency, then use that energy to run an electric motor with 95+ efficiency. especially when the battery is lightweight with Lithium or NiMH than to carry around a bunch of water.
those that claim you do are snake oil salesmen