Just....do both? Have a proper, city-size, self-sustaining colony on the bottom of the ocean, and at the same time keep developing rockets and vehicles to bring as much material and and many robots over to Mars as physically possible. Like, I don't think getting to Mars is an issue - we probably could send and even land someone on Mars within a couple years if we absolutely had to. But then the question is - what then? That's the hard problem.