And ice can also quickly sublimate in a vacuum depending on the temperature, and during the day the Moon temperature rises above the boiling, let alone melting point of water. The Moon's gravity is too low to retain water vapor, so as soon as you get liquid water it disappears within minutes. Meters of ice would sublimate in a day. If the Moon had a 3 km thick layer of ice, like we do, it would sublimate in 10 years.
You don't need heat or impacts, just some basic knowledge of thermodynamics.These well-known facts have been around for over 100 years.
You would have to have ice in permanent shadow at the bottom of a polar crater to get water. But, on a side note, we already sent impactors to the most likely place where water could theoretically exist and did not find any.