Before Mars is a viable colonization destination, we need to build a good size habitat that's fully or almost fully self-dependent in one of our many oceans that make up most of this planet. If the purpose really is to attempt to escape extinction events I'd rather live down where food and water are plentiful and within arms reach than any celestial object. Additionally, international waters are by definition not owned or controlled by any government and are accessible by anyone with a boat.

Technically, this is more than possible with current tech at less cost than sending a rover to Mars (probably), but no one wants to do it because it's not land you can plant a flag into and "claim" as yours.