you could use artilley as a coastal bunker, cost 4, defense 1, attack 1 / with the rocket development
they can not be destroyed, and can be used by invading force’s, build as many as are needed, stack control marker’s to know how many coastal bunker’s that are at a territory,
i do not think that your idea would throw the game out of balance, it could lead to a lot of sunken ship’s, with the artillery hitting at 2, or less coastal bombardment, with player’s avoiding the territories that have a heavy stack of artillery,
it could also work to the attacker’s advantage with artillery used for attacking ship’s,
the bunker’s could be upgraged to attack invading amphibious assault forces,
although i think the cost would have to be revised
the defending player could choose to disable a bunker as a combat casualty,
amphibious assault’s could be conducted as a prelude to the invasion,
if the invasion would be successful, the attacker would take control of the bunker’s,
if the defender would be successful, the bunker’s would be operational at the start of the next turn