Help me out if I am missing something.
I think the only thing you are missing is an understanding of the research.
You seem to be aware that during formation of the terrestrial planets volatiles were relatively depleted, so although water is a commonplace compound in the universe it was deficient in the formation of the Earth. (You noted: The universe is comprised of Hydrogen and oxygen and, so many other elements that comprise earth and all of our other planets throughout our universe. It all depends on how far they are away from a sun
.) This depletion seems to rule out the notion that the oceanic water was produced largely by degassing of the Earth's interior.
The alternative explanation was that the water had been acquired through impact by many bolides (comets or asteroids) from the formation of the Earth through the Late Heavy Bombardment period. Please note that a single comet or asteroid is not seen as the source of the water, but rather very many such bodies. (A really big number. Work it out - simple arithmetic.)
You ask where the comet or asteroid would have acquired this water. They would have formed sufficiently distant from the proto sun that water could condense, along with dusty material, and accrete into small bodies, either asteroids (with hydrous minerals) or comets (which are still reasonably called 'dirty snowballs').
Arguments as to whether the bulk of the water came from asteroids or comets have swayed back and forth. This latest research points in the direction of comets.