I'm trying to justify making one of the larger ones, but that would mean making a whole new board to play on...
You don't really need a different board. A large blue tarp can be effective for sea. You just need to make strips of coastal terrain to put between your usual terrain and this "sea", and then you can play large ship movements along the coast - and on open sea if the table and tarp are larger.
Example (for 14th C. European medieval):