Whether or not your fuel supply needs to be deuterium or protium shouldn't
make any difference in the amount of water taken from the ocean. If you
only use the deuterated water, you'd put the rest back anyway. So you
can do your calculations (liters per year) without regard to the level
of "enrichment" that your fuel needs.
As for replenishing this water, it wouldn't be too hard to capture a comet
or two and drop chunks of snow into the atmosphere. A well-managed starport
probably includes provision for environmental replacement, including
replacement of ocean water.
Martin Snow
[email removed]
[email removed]