But the issue of moving water from earth to space then building a 2 layered craft strong enough to surrou d passengers with a foot or more of water. While doable theoretically. Is just a huge huge task.

If mankind is seriose about such. Robotic collection of ice from space is more practice. Moving it towards earth using it to create hydrogen and oxygen slowly via solar. Then using that to move the water itself into earth near orbit. From there building a vessel able to rotate and protect occupants from radiation etc would be possible.

But seriosly the amount of work involved. Mars really is not the best place to go. Once humanity has developed the ability to generate graverty. Confirmed mankind can live is centrifugal generated enviroments. And developed to robotics to move ice and asteroids into orbit.

Honestly building O'Neil station like structures would be healthier then trying to live in 1/3 g of Mars anyway. Mars is worth learning about. But musks population plan is not really the best way to make humanity less at risk of planetary loss.