Harbor

A harbor is created automatically in towns that are close to a body of water. Any town with a harbor has and  as available commodities.

It also gives additional options for trading and barracks upgrades.

Trading
A harbor allows traders to travel by ship to any other town with a harbor. Especially in early game traveling speeds by ship is much faster than by basic roads. Beware of pirates though.

Combat ships
In a harbor town, the barracks have an additional upgrade: a shipyard. This will allow the construction of both combat vessels as well as colony ships.