The Antarctic coast and particularly the western peninsula would be able to sustain small human communities, even without modern technology (like Greenland or Northern Canada can). The interior is more forbidding, but nothing like Mars.
As for the sea and seabed, the logistics of sending supplies are not even comparable. It's actually a counter-example. If living on the sea or on the seabed has not been viable, what would make outer space viable? For a few 10x millions you can probably get a floating greenhouse and subsurface habitat in place. Mining the sea bottom is not yet valuable, despite many more mineral resources available.
As for the sea and seabed, the logistics of sending supplies are not even comparable. It's actually a counter-example. If living on the sea or on the seabed has not been viable, what would make outer space viable? For a few 10x millions you can probably get a floating greenhouse and subsurface habitat in place. Mining the sea bottom is not yet valuable, despite many more mineral resources available.