One big gap here - where are you going to get the deuterium or tritium? These isotopes are very quite rare, but on Earth, we have so many hydrogen atoms just lying around in the form of water that we can afford to process 100kg, a bathtub full, of water to find half a gram (quick estimate) of deuterium atoms.

Also, we really don't know for sure how much water there is, and how uniformly distributed it is (probably it's mostly at the poles).

All of this just makes your argument stronger, of course.