What? No "Z" tert?
Maybe Z could be a ship in the ocean that attacks the 6 coastal terts (A, B, C, N, O, R) -- but can only be attacked from the non-swamp coastal terts (B, C, O, R). It would not be part of a continent bonus, e.g. like the "Rio de la Plata" tert on the Triple Alliance map.
I like a ...