I know people have had ideas for "sea walls" before and the ideas were struck down due to designs.
I do think we should have some sort of coastal defense system, advanced ancient civilizations had towers and walls for defenses.
The suggestion I have is to give us tower defenses that we can build on land next to the ocean.
We can build 8 individual towers along this wall. Max level of 48. Damage, accuracy, munition amounts, garrison size, armor and hit points scale with levels.
Levels 1-15: Towers have Catapults.
Levels 16-32: Towers have Ballista's.
Levels 33-48: Towers have Mortars.
Towers can be attacked by ranged ships only. Once towers run out of munitions the town owner loses the battle.