Alternative Fuel

Sometimes it seems I just have to be the one who asks the stupid questions; or so it would seem.
In discussing alternative fuel for cars tonight I just had to open my mouth and wonder out loud what the long-term ramifications might be of both of the options we were discussing.
One was hydrogen fueled cars, which would output water. The other was water fueled cars that would output oxygen. So here is how my head works:
I'm thinking, 'Ok, the car runs on water and puts out oxygen. But humans need water and it's already in short supply, so there's one concern. Even if a car gets 1,000 to a gallon of water, with millions of people driving millions of miles, we are using a lot of water that we need to survive more than we need to drive. Unless it's salt water, which we don't need to drink, but do we need it for something else? Is the salinity of the aquatic environment balanced ever so delicately that we could cause problems with something like this?' So, I made the mistake of asking the question 'what might the ramifications be?' out loud, minus the thought process. The consensus seemed to be that because we breath oxygen, there couldn't be too much of a good thing. However, I know that we don't breathe 100% oxygen and that too much oxygen can be dangerous. Babies (mostly preemies) were blinded because they were given 100% oxygen in hospitals after birth. Well, that and the silver nitrate parents were assured was safe; until it burned their babies eyes and made them blind. Just more stuff in the child-bearning year that people assumed was safe because doctors did it, although nothing was actually scientifically tested or supported by evidence. Such as DES, x-rays in pregnancy and thalidomide in the past, or cytotec (or inductions in general) and ultrasound today.
Anyway, at the time I couldn't remember how much oxygen we actually breath (21%) or was too much though, so I didn't pursue it. However, since I'm not a scientist, I was curious to know, does oxygen build up in our little bio-spheric bubble, or does it dissipate? Is there a o2 saturation point at which our plants will no longer thrive? Does more o2 in the atmosphere change the way rays from the sun are filtered? You know, stupid stuff like that.
With the hydrogen car that creates water, the first thought is 'great'! We need more water. At first it could solve a great many problems. However, the planet seems to have a very delicate balance with a great many things. Perhaps there is a point at which the planet becomes too wet? What happens to the way light filters, or a myriad of other possible issues, if there is too much water in the atmosphere? Isn't that part of the problem with greenhouse gases heating the planet? Too much of our water ends up in our atmosphere through evaporation from the heat, instead of on the ground where we need it?
Now, I'm not against either of these technologies of cars that run on benign substances. I was recycling when we still had to take our stuff to a recycling center. I used cloth diapers and breastfed, not just for the health of my child, but for the health of the planet. I had a low-flow shower head in 1988 and friends and family complained about what a nut I was for conserving water. So, all I'm saying is 'what if'. It's the way I think. We keep tinkering with the balance and sometimes the ramifications aren't thought through very well. Water and hydrogen are certainly better than petroleum products, but are they harmless in the massive quantities that would be generated if they were a fuel source? That was all I wanted to know. It seems it's stupid to wonder about such things though.
So, I'll get back to wondering stupid things about birth again.