I’m not sure how one dates water, but most of the water on earth is now believed to have come from comets which collide with earth on a regular, but diminishingly frequent, basis.
“I’m not sure how one dates water[....]”
You date the age of the water molecules by counting the ratio of the Oxygen atoms with decayed isotopes of Oxygen versus less decayed isotopes of Oxygen. The older the Oxygen atom is the more likely it will contain a stable isotope of Oxygen. The younger water molecules are much more likely to be composed of Oxygen atoms which have not yet had enough time to decay from an unstable isotope of Oxygen to a stable isotope of Oxygen. By counting the ration of these isotopes of Oxygen, you can see the relative differences in the age of the Oxygen in the water molecules.