[ethbr1]

You can pull power and air out of water, given enough infrastructure. Mix gases would probably be a pain in the ass, depending on what they're using.

Not sure what aquaculture at greater than sunlight depths looks like, but assume you could farm the upper layers of the water column.

The biggest limitation to self-sustainability is probably interior volume, given the need to have manufacturing chains available.

[bruce511]

All this is true, but none applies to the project under discussion.

As an aside, I expect any kind of "backup humanity" goal would almost certainly be easier, more sustainable, and more likely to succeed on land than under water.

On the other hand if we messed up badly enough to require a backup, I'm not sure we deserve it.