alot of high powered NA cars do run water meth for a hell of alot more time that you guys do on boost,
however I don't believe he said that it splits the water into Hydrogen and oxygen and it wont as you require a few other factors going on, like electrolysis.
Yet as for the increase in comp, it depends on where abouts in the cycle your talking, on the actual compression stroke you would prob see a decrease in compression as the water absorbs a hell of a lot of heat and cools the mixture making a lower pressure area, however then when you are on the combustion stroke, the water is heated way beyond the point it will remain as water and will become steam and at this stage you will have an increase in compression, im not 100% sure on if the net result will be an increase or decrease in compression overall and i really cant be arsed trying to work it out.