On a 16km X 16km map, I think helicopters can be made in if it is to be realistic. And river ferries are a really good choice to add as well, since I doubt a 16kmx16km area can contain so much of the seas, and large ships never transport goods around in such a short area. Neither decent-size planes nor ships go on short trips.
River ferries are different things though. They should be added since river transport is an important transport method, can legitimately work in any area whether large or small, and makes for an interesting gameplay aside from land-based transportation, sometimes even creating demands in places where land-based transport is non-existant. For example taking passengers from a mountainside station where railways cannot go any further, moving them a few kilometres ahead and dumping them on the plains where they can board a new train and continue their journey, also probably making a new town to grow up around the place.
Of course, all that can be a DLC for those who want it, making it optional those who don’t want to see them in their games.