Never mind all of that, im pretty sure that it takes a shit load of energy to split water into hydrogen and oxygen.
Think how much energy it takes to boil water. Thats just converting it from liquid to vapour. Physically breaking the hydrogen oxygen bond requires loads more energy (i think)
I would get out my chemistry degree books and work it out, but i cant be arsed (and probably wouldent be able to work it out anyway after all this time out of uni).
The extra weight in batteries, drain from a massive alternator and weight in water would likley override all the benefits (thats why Ford, Vauxhall etc havent done it already)