Is Solstheim part of Tamriel worldspace? in other words, can you go there swimming, and Skyrim worldmap has been enlarged to include it, or it's a different worldspace (a sort of paralel dimension), with a different worldmap?

Different world space with a different map, haven't tried swimming back though, might be possible.

Yeah, it's a separate world space with its own map as far as the game mechanics are concerned and swimming back is not possible, but "parallel dimension" isn't the right way to describe it either.