Well, I don’t exactly know how our water got here, but this I do know, God made it and He put it here on our round planet. Exactly how He might have done this is a fascinating question, and I think it wonderful that our scientists are slowly, sometimes with missteps, unlocking the secrets of the universe.
You said it faster than I could have. Just found this thread.
Methane, carbon dioxide, water vapor and ammonia contain all the ingredients necessary to create an atmosphere of some 80% nitrogen, about 20% oxygen, and a smattering of other elements and other gaseous compounds.
All it takes is the presence of some means of converting the combined form of oxygen into a free molecule of diatomic oxygen, and breaking down the carbon dioxide. This process of breaking oxygen free and taking up carbon dioxide has to be on a long-term sustainable process with a renewable and unending source of energy to keep the process going, of which this planet is blessed with having available.
Once the oxygen is free, it reacts with the ammonia to form water molecules and diatomic nitrogen molecules, a process that had to go on a long time to reach the levels now known in earth’s atmosphere.
Now, there is an awfully large quantity of water on the earth, but since hydrogen is by far the most plentiful element in the Universe, and oxygen is the second most reactive element, the formation of water was almost a certainty. Given conditions of temperature, pressure and time, much of this water was already in the elements that made up the primordial planet earth, and as the planet cooled, through radiation, and contracted, expelling considerable amounts of gaseous material from the interior, the total amount of liquid water continued to increase. We did not have to wait for gaseous comets to strike earth, most of its water supply was already here, but as water vapor, or locked up in mineral compounds.