I know I have seen this recommendation before. But I would like to give some more reasons to why this would be a great addition to the game.

So basically, this is how I think it would work. The naval gun would have the range of maybe an artillery lvl 1 or 2 and would hit any enemy ships and possibly any ships with the fire control button. The naval gun would be stationary (obviously) and do decent damage to ships but not very much to anything on land. Also the guns should have decent AA as well. They should only be able to built on coastal provinces. One thing that may cause a serious problem is that convoys would get torn to shreds while debarking. So in order to balance this, the naval gun should be expensive and with less health, the less damage it does, similar to land units.

So why should they be made? Well firstly, it would make the smaller islands extremely dangerous. In WW2, many operations were held on standby until the nearby islands were captured as the ships passing by them could be sunken due to the guns on the island. Adding naval guns would force the player to do the same thing as the small islands that no one really cared about before could become extremely dangerous. Island hopping would become a reality and the naval game, a part of the game not as fleshed out as the air or land, would get more serious. Strategic bombers may also become more useful as they would be able to destroy the guns rather easily.

Artillery may do the same job somewhat. But they:
A. Do not do enough damage to ships
B. The player is very unlikely to have artillery everywhere on the island