Colonizing the bottom of the ocean would have many of the same practical benefits of colonizing Mars, as well. It won't save the human race if the entire planet

Colonizing the bottom of the ocean would have many of the same practical benefits of colonizing Mars, as well. It won't save the human race if the entire planet is destroyed, but short of that it offers some protection from most of the other existential threats (disease, nuclear war, asteroid strike). That being said, it's not clear what would sustain an undersea colony. Only a fully self-sufficient geothermal-powered setup would be properly effective as species-insurance.