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 or more ports, a sea route will form as long as they are close enough (4 intervening sea tiles or fewer, including routing around land if necessary)(Resets after going under a port). Roads and sea routes can join together at ports 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.