Perhaps there could be a building that essentially acts as a coastal battery unit. It could have a range of 150 and have naval radar to detect and destroy oncoming enemy ships. The higher level the building, the stronger it is against ships.
to prevent overuse, it could be built only along every other province (i.e. if province a is next to province b with a coastal battery, you cannot build a coastal battery on province b)
to prevent overuse, it could be built only along every other province (i.e. if province a is next to province b with a coastal battery, you cannot build a coastal battery on province b)