I can't be the first person to have figured this out, but since I have, I wanted to share it with the other slowpokes who didn't get the memo.
Very simple way to generate lots of purified water.
-Claim and clear the starlight drive in. Especially, scrap the barrels from the pool in the middle to remove the radiation hazard.
-Set up an industrial water purifier in the pool, and a medium generator.
-Surround the pool with fences and whatever defense you feel you need. I used 9 turrets on platforms inside the fence. (I'm not sure if this is necessary, since as far as I've seen, settlements with no people do not get attacked. If anyone knows better, feel free to tell me.)
-A day later, go pick up the 29 purified water at the workbench- at least that's what I found there.
-Do NOT set up a beacon or any beds. Keep the population at 0. Since there's no one there to drink it, all of the output of the purifier goes into the workbench. I haven't yet worked out whether it stacks, or if you have to empty it daily, or any of that sort of thing.