I’m not sure what level you are, but the town map already has all the buildings. We can open them as we level up. The fishing pier is leveled up using coins and game commodities (stone, lumber, steel, etc). The maximum is 5 boats, but the number of fish they collect increases as we continue to level up the pier.
i have not seen anything suggesting Seaport has plans for more buildings or more fishing boats to be added to the game any time soon.