I thought maybe ships are bound to provinces like landunits. Rhodian ship in Rhodes etc.. But that does not normally seem to be the case.
On checking EDB:
recruit "generic ship navas" 0 requires factions { macedon, romans_julii, thrace, numidia, }
Indirectly, they are bound to province. The ships you can recruit are determined by the level of military port, which is bound to provinces.
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