That’s true. Local water, even as trace ice crystals, would be easier to harvest than chipping apart a comet in deep zero g. But ultimately, your materials for both construction and life support are going to have to start coming from space, and asteroids and comets are the obvious choice.
The best strategy would probably be to send a relatively small vehicle to the comet (small relative to the comet), something like the power and propulsion core for the new lunar gateway, essentially just a big ion thruster with a bunch of solar panels. This can push the comet into an orbit that swings it by the moon to capture it into an earth orbit. You may need to do some earth flybys to lower the comet’s orbit first, so the mission could take years. But to make up for that, comets are huge, and after it’s done you have a source of many different materials to work with right here in earth orbit, enough material to last decades or more.
But not all water is useful.
We have a lot of non useful water on Earth in the oceans that has too much salt.
Water from non-Earth sources might contain dissolved minerals at poisonous levels for agriculture, much less human consumption.
And if there’s liquid water from a non-Earth setting, there might be some kind of unknown exo-organisms living in it.
Water from non-Earth sources might contain dissolved minerals at poisonous levels for agriculture, much less human consumption.
Oh yeah, it’s practically guaranteed to contain nasty stuff! We’re gonna drink it anyway though.
Most of that water on earth that we’d consider “not useful” would fall into the “100% useful” category if found in space. As long as the contaminants have a different boiling temperature from water, you can always boil the water into steam in order to separate it. Or you could also use electrolysis to separate out the hydrogen and oxygen and then recombine then in clean tanks.
These are expensive methods of purification, energy intensive, but solar panels really well with no atmosphere and 24/7 sun exposure, so this is all feasible.