Along the lines of what you have proposed:
I am not sure about the suitability of having ports have any defense, but rather coming up with a unit that can adress a number of characteristics of a fortified territory (think maginot defenses).
REPAIR DAMAGED BATTLESHIPS
Battleships are powerful units. A damaged battleship may be repaired if it is in a sea zone adjacent to a territory with an IC controlled by its country. The repair costs are equal to the roll of one d6. The battleship may not move or participate in combat during the turn of repair. The repair is complete at the end of the players turn.
Fortresses
These units can be added to any controlled territory for the cost of 10 IP and can defend only with two rolls each combat round with a defense value of 3/3.They “protect†the defending units from direct attack. The units inside the fortress cannot be affected by attacking combat rolls until the fortress is destroyed by a roll of 1 only. Once built they cannot move and if destroyed cannot be reused by enemy forces. The maximum number of fortresses is limited to the IP value of the territory and in the case of China is limited to one.