I think you are putting two inequitable objectives together.

The primary goal of interplanetary colonization isn't to save the world. There might be some selling points to "save the species," but we aren't really at that point. I view it more as a challenge to see if we can, not that it's a necessity.

It's "cool" to be able say the human race was able to colonize multiple planets, and to be apart of the team that accomplished the feat.

Why would they want to colonize the ocean floor, when there is still plenty of land to develop?