Have you seen what happens to people who live in zero gravity for long periods of time? It doesn't sound fun. I do get the appeal of wanting to experience it for a short period of time, though.

I would also say that scuba diving is probably the closest you can get to experiencing anything close to zero gravity on Earth.

Sure, living in zero G for a long period of time is not good for you.

You asked how space was different from a submersible. I gave you one difference, you expanded on that to a second.

Any space colony intended for long term habitation would create artificial gravity through centrifugal force. It's completely doable using materials sourced from the Moon and near-Earth asteroids; there are design studies dating back over fifty years.