The high concentrations of deuterium reflect the initial composition of the subcrustal oceans. The organic matter came ''from'' earth and was never introduced ''to'' earth by the comets.
A back-tracking of cometary orbits by Dr. Walt Brown places comets Halley, Brorsen-Metcalf, Swift-Tuttle, Pons-Brooks and Olbers all near perihelion in 3344 BC (±1 year). He proposes that this was the year of their ejection from earth. (http://www.creationscience.com/onlinebook/FAQ212.html#wp7530922)
Further, an analysis by Stephen Kelly indicates that of those 5 comets, four of them have orbits which currently intersect Earth's. All four cross Earth's orbit heading inbound toward the sun at a point in the Earth's orbit that would indicate mid-to-late autumn if they were, in fact, ejected from Earth in the Great Flood.
Halley 10/30
Brorsen-Metcalf 9/10
Swift-Tuttle 12/1
Pons-Brooks 12/11
This is consistent with the Biblical indication that the Flood began on the 17th day of the 2nd month (Genesis 7:11), starting at the autumnal equinox. These comets, upon ejection from Earth, fell toward the sun and were slingshotted into their current orbits.
==== Problems for the hydroplate theory ====