unfortunately they do not.... The electrical energy required to produce H2 and O2 from water is extraordinary (if you expect enough to make a difference....Consider if you will.... A 1.3 liter engine inhales 1.3L of air for every 2 revolutions. at a moderate speed of 2000 rpm the machine inhales 1300 liters of air at normal pressure every minute. Some of the best water cracking devices will produce 3.9 liters per minute. that's roughly .3% of the total air charge, not to mention that you are using about 40 amps at 12V to power the separation.
All in all its a useless propositon at current level of tech.