Just got Vintage story this week, haven't been playing long, but from what I gather, saltwater's kind of a super new development. Salt being huge in food preservation, I feel like you should be able to precipitate it out of ocean water, be it through historically accurate basin flooding techniques or just cooking it in a fire. Might already be in-game and hasn't been documented yet, but reading the wiki
(I feel like playing this without the wiki is a little masochistic lol) I saw "Halite is the only current source of salt in Vintage Story" and figured I should mention something.