I'm more for a seaport built by settler/engineer as airbase (on a shore, of course) inside a city radius.
This will let the player to add a port to a city just inland: I'm thinking of great cities with port but not exactly on the shore: London, Rome (in the past), etc.
This idea can be another way to partially solve the canal problem (see the related thread).
Port will repair ships, can be conquered by ground or sea units, same for defense.
I think it can add tactical and strategical possibility, also about city placement.
I bet it is quite easy to implement too (AI use of it apart, maybe), so Firaxis don't need to spend too much developement time about it
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Admiral Naismith AKA mcostant