You would think we would get really good at the latter before going after the former, and yet I see no interest from people wanting to live in a (shallowly submersed) submarine. It would also be an order of magnitude less expensive and dangerous.
If we can't build, say, a 10K-person inhabited city on the south pole, how can we even imagine we can build it on the moon or Mars?
Just off the top of my head:
- different ability to re-stock
You could re-stock your submersible just about anywhere. You're going to have to do a lot more planning for your groceries when you go in space though.
- access to microgravity
This simply isn't available in a submersible. Microgravity provides some interesting manufacturing and biological capabilities.
- completely different pressure profiles
Combining different pressure environments in microgravity is particularly interesting to me.
- different instrumentation capabilities
It's not just the view -- the atmosphere plays merry hell with instruments when measuring the cosmos. And it does so in ways that just aren't relevant to underwater environments.
I would also say that scuba diving is probably the closest you can get to experiencing anything close to zero gravity on Earth.
You asked how space was different from a submersible. I gave you one difference, you expanded on that to a second.