Magic happens..there's no such thing as free energy
So, where does the energy come from to split water into hydrogen and oxygen? ... your alternator, which is powered by your engine. So, unless magic happens, the energy going in is more than the energy coming out. Secondly, if you use acid to make the water easier to split, you need to have water traps to remove the acid from the hydrogen, otherwise the air intake corrodes up. Maintenance on this is critical, if you don't want to wreck your engine.
One of the claims is that it makes the fuel burn more completely, similar to gas injection mixes. These typically use 1.5 to 2% mix (? from recollection), so that's a lot of hydorgen to create.
I agree with all of the other sentiments against hydrogen, but the plumbing from the seat to the engine is a bit tricky.