re: Should we be building colonies under the sea?

Humans evolved to do well on land in a breathable atmosphere. The only "colonies" we'll be starting underwater or in space will be run by our robots.

Robots that we'll want to program to be more able to make decisions on their own in order to serve us better.

Here's one of the earlier models:

No, they're focused on building colonies on Mars right now. One way ticket only.

By the time we get to where we can colonize anything we'll have prolly already reached the point where we can and have uploaded our own consciousness into a computer so making a place hospitable for the human body may be a mute point.