Ports are buildings that can be constructed after you have researched the Sailing technology. Ports produce 2 population to the city it is built in. They cost 10 stars and can only be built on water tiles that are in your territory. Ports will also convert (non-amphibious) units that move onto them into boats.
Sea Routes Edit
When you have 2 ports or more, a sea route will form as long as they are close enough (4 intervening sea tiles or fewer, including routing around land if necessary). Roads and sea routes can join together to connect cities to your network and give +1 population to the connected city and the capital. If a port is adjacent to a city, no connecting roads are needed. Before the "Power" update, sea routes did not have a tile limit.