Hmmm, where do you get that? NASA? Way back when I think it was Fred Whipple who said comets are dirty snowballs. Are you making a distinction between water as ice/snow vs liquid water(phase change)? Obviously comets in space would have water in solid form, it then sublimates into the tail as it gets warmed by the sun.
But positing that 1 to 2/year comets hit the earth and ONLY the earth in a barrage from 4.4B to 3.9B, containing 1 cubic mile of ice on the average, to make our oceans is nonsense. Impact-splash as a theory is a right brain/no-brainer. It can’t explain how our OCEANS got here so early on. Genesis 2:6 gives you a clue...
Now we have evidence and it does not support Fred.
So far we have flown by three comets and bopped one on the nose with a 643 pound chunk of copper and in all four encounters the ONE thing we have not found is H2O.
In addition, a recent comet passing fairly close to Earth had one of our X-Ray telescope satelites look at it and astronomers were shocked to find it was emitting X-Rays. That does not fit with the passive ice sublimation model at all.
Other comets have flared up far beyond the distance where the sun could heat them enough to cause sublimation, one at twice the distance to Jupiter!
In the Deep Impact mission to Tempel I, we did find both Hydrogen and Oxygen but only in the form of the Hydroxyl radical, OH, and that was at a concentration less than 1/100th of the amount of H2O they expected to find. The OH content released was about the same concentration that would be found had we slammed a bunch of copper into a rocky asteroid or meteor.
To cover up this lack of water, and to preserve modern cosmology so they wouldn't have to rewrite all those textbooks, NASA and the Deep Impact team ignored the evidence and held a press conference six months post impact to announce the had found water... or at least the elements that make water... and since the "water" was not enough to support the accepted "dirty snowball" theory of comets, they had decided that comets were really "snowy dirtballs" and that they had just not hit deep enough to release the water they really "know" is there. This is modifying the facts to fit the theory.