[Mike Darancette]

I really, really doubt this hypothesis. It would require millions of comets to provide enough water.

A million comets is 1 comet every 1000 years for a billion years.

[Slicksadick]

Crude estimates of the brightness of small comets which are devoid of brightly glowing dust particles are indeed about the brightness of Venus, and their impact rate is about 10 million per year. It is remarkable that the orbits and speeds calculated for many of the "fireballs" are similar to those for the small comets.

From here...
http://smallcomets.physics.uiowa.edu/lecture/lect5.html

Dirty snowballs the size of houses, by the millions.