"Will they go to the ocean tomorrow?"
Translation:Wataenda baharini kesho?
No, as far as I know, it's nothing (or little) to do with specificity. The -ni suffix transforms a noun into one of the three locative classes ... but which class is meant is unclear unless there are other words in the sentence that show you which one.
Class 16 (pa, pangu, pako etc.) is mostly used for precise locations. "To, at, in, on, from" (Is this what you were thinking about with specificity??)
Class 17 (kwa, kwangu, kwako etc.) is generally used for less precise locations. "To, towards, at, in, on, around, from" Class 18 (mwa, mwangu, mwako etc.) is used for internal locations. "Into, in, inside, out of" ...
It's not quite that clear-cut because there are idiomatic usages of each. The word mahali/mahala/pahali/pahala (various variants of the word for "place/location") is always locative without the ni suffix and it's usually (but not always) class 16 (eg. mahali pangu = my location).
In any case, which of these classes is meant is unclear in the sentence because there's no word that can show it. The -ni simply says this word is a locative noun tha belongs to one of them. After the verb kwenda, the destination is almost always either a locative noun or a prepositional phrase (introduced by katika, kwenye, ndani ya etc.) ... I've seen exceptions like Ninaenda shule or Twende kazi but mostly the -ni simply indicates a location.