I don't know about the water, I feel like there is more information we don't know here. What I do know is that vents are a good start. Simple roof jackvents are easy to install and don't cost very much. There are several great books that illustrate how to do this, and probably even a site or two on the internet. Please be careful on the roof.
As far as more information, it would be good to know where you are climate wise, what temperature humidity conditions exist when this happens, what is the ground like outside, does the ground slope AWAY from the garage at least 10 feet, do your gutters go to a foundation drain or at least splash blocks, are you getting water in under your garage door, etc. Is your water heater leaking?
Like I said, it could be lots of things, but ventilate your roof, you're cooking it and reducing its life.