I would call this a bug, since it affects gameplay negatively. If an enemy has a boat stationed in it's coastal city, you can bombard the city turn after turn and the only thing you will affect is the ship(s) that is(are) docked there. You take it down a few hit points, and then the next turn it it completely healed and the cycle continues. This proves to be an effective anti-bombard defense for whoever wants to use it, (AI or human) just put a few ships into the city and voila, your defensive units (and city improvments) are not vulnerable to bombard. I hope this gets addressed.