I'm seriously concerned about the naval implications of one unit/tile approach, particularly for aircraft carriers, but if I were a game designer faced with such a constraint, I'd approach it this way: 1) a harbor/seaport would be a tile improvement within the city radius, something like our present fort, but it would have the effects of a harbor city improvement. Rome wasn't built on the coast, but it had a port to serve it. This would allow units to embark without disrupting the city garrison. Same with airports. 2)I'd replace workboats, galleys, transports, etc. with a unit called a " merchant marine"- the seagoing equivalent of a worker. The merchant marine would serve as a military transport, and could be closely escorted. In peacetime it could be used to work sea tiles, But rather than build a net, it could run for port when enemies or barbs approach. I'd also allow them to work explored tiles outside my territory, to simulate whaling, and diplomatic issues over fishing rights. I'd also allow them to automate as an additional trade route between two particular cities, providing both extra commerce, and a capture/pillage opportunity for privateers. This way it would still be possible to conduct amphibious assaults and to drown an enemy army rather than burry it.