Living on a ship at sea, or underground / underwater, should give enough protection against whatever post-apocalyptic situation exists on Earth, it's pretty much always going to be more survivable than Mars or outer space. You might need a protective suit, air filters etc. Still 100x easier than having no atmosphere at all.
What if the threat is not environmental collapse but hostile government?
If they can reach your submarine, they can reach your space colony.
Surprise Dodgeball reference. But I don't think the ocean is as far away as deep space. I also think due to its proximity, it could be entangled in political paradigms that lead to entanglements in international conflicts and the independence from those political paradigms affords a form of insulation.