Can you really mine water in interstellar space, how? there’s not that many asteroids just floating out there, I think.

Maybe the previous poster was referring to the Kuiper Belt and/or the Oort Cloud? There's certainly plenty of water ice out there, with no particularly deep gravity wells to worry about.

Collecting it while usefully en route to another star system seems like it would be tricky from a delta-v perspective, but maybe the solution is to:

1) build a small worldship from an appropriate asteroid 2) fill your internal ocean in the outer solar system over the course of a generation or three 3) dive back down the gravity well for a gravitational assist from the Sun as you head to your interstellar destination 4) en route, slowly use part of your ocean as propellant, while living inside the rest of it as shielding

	▲	m4rtink 3 hours ago \| parent [-]
		There is a nice concept I've read I think on either Orions Arm or Project RHO about basically settling the Kuiper Belt and the Oort Cloud. It would not really be about primarily traveling anywhere, just people settling this are with its immense resources over many generations, until possibly the ones on the furthest reaches would be able to make the jump to the next systems Oort cloud & slowly settling it down the gravity well, maybe even reaching the inner system eventually, provided something so tiny and sweltering would be of interest for them by that point. ;-) IMHO this is actually much closer to how humans spread along the Earths surface. It was no like someone just get up and walked from Afrika to Australia. The various human group just expanded their habitat over time until they spread all over the place over many thousands of years and this would be similar.