On Science Daily we found an interesting article on the source of water on the Moon. A significant amount of lunar waters exist in the form of hydrated minerals and computer simulations show that asteroids are a more likely source of this water than comets. Comets contain water in the form of ice, while asteroids do in the form of hydrated minerals.
According to the article the impact velocity of a typical comet would cause most of its water content to evaporate. On the contrary hydrated minerals from asteroid impacts would be absorbed into the Moon’s regolith.