I was imagining seawater left out to dry in the sun or artificially with heated copper rods. The water evaporates, leaving behind the salt. It was either that word or Seamining, because there are such things as landlocked salt mines, and i chose the one that alliterated. Salt is a pretty useful little substance, especially in a port town. Fish goes bad, but salted salmon or mackeral lasts for weeks on the back of a trader's wagon, so a port always needs salt. Although i don't think many dwarves that aren't valley hicks would take to the fisherman's life.