Rosetta’s comet ‘sweats’ two glasses of water a second
esa.int
esa.int
Besides, I wasn't so much meaning going as far as the Oort cloud, was just thinking you could tag along with a comet for a bit as it went through the mid solar system and suck up some reaction mass from it without having to land on the thing.
If you have practical hydrogen-hydrogen fusion, it is at least theoretically possible to sustain quite a lot of civilization in the Oort cloud, in terms of raw energy and atoms. Without fusion it's hard to see it as anything but worthless ice, though.