The Harbor is a specialty district in Civilization VI focused on naval power and maritime trade. It requires Celestial Navigation and must be built on the coast.
- Effects:
- Receives Major Bonus (+2
Gold) for being adjacent to the City Center, Standard Bonus (+1 Gold) per each adjacent Sea resource; Minor bonus (+0.5 Gold) per each adjacent District - +1 Great Admiral point per turn
- Adds 1 Trade Route Trade Route capacity, if city has no Commercial Hub
- Allows for sea lane Trade Route Trade Route (even if the district is still under construction)
- Allows its parent city to build ships, even if the City Center is inland
- Newly built ships will spawn at the Harbor tile (as long as the Harbor tile is unoccupied.
- Removes movement penalties for units Embarking to and from its tile (even if the district is still under construction)
- Allows its parent city to build Ships requiring Strategic Resources with only 1 count of the relevant resource
- When the Seaport is built, the parent city may construct Fleets and Armadas.
- Buildings grant experience bonuses to ships built in this city
- Specialists add +2 Gold and +1 Science each
- Pillaging Harbor yields Gold
- Receives Major Bonus (+2
Buildings
The following buildings can be constructed in a Harbor:
Projects
- Harbor Shipping: Earns Gold equal to 15% Production used and Great Admiral points
Strategy
The Harbor is one of the more important districts in both single- and multiplayer games. The most important bonus from it is +1 
Trade Route Trade Route capacity, but the extra 
Housing Housing from its buildings is also a great bonus. The extra 
Food and 
Production from domestic Trade Route Trade Routes are important early game boosts.
As sea tiles are often barren in Civilization VI, one should avoid directly settling by the coast. Here is where the Harbor comes in handy: it allows for ship production and sea trade while allowing players to place the otherwise coastal city in-land. This helps players access in-land resources, fresh water, and a greater number of tiles for districts for their "coastal" cities. That said, maximizing the Harbor's 
Gold output requires building a city on the coast, as the Harbor gets an adjacency bonus from being next to the City Center.
For ship production, the Harbor fills the role of an Encampment for ships. The Harbor also allows for the construction of many coastal Wonders, which is something to consider for single-player games.
Between building Commercial Hubs and Harbors, whichever gives higher adjacency bonus at the time should probably receive priority. Both districts can also make a nice combo, especially when combined with the City Center, since Commercial Hubs receive adjacency bonuses for being next to a Harbor and Harbors receive adjacency bonuses for being next to a City Center. While Gold is not the most important resource, extra early Gold production is still helpful. It is also nice to have at least one Harbor for sea exploration (in case you didn't settle any cities on the coast). Map discovery via sea can be important in single-player games, particularly with continent-heavy maps.
It is possible to construct Harbors on Lake tiles! Although this doesn't make much strategical sense in terms of constructing ships and exploring, you will still be able to use the district's secondary bonuses mentioned above, and even construct sea-based Wonders (if the lake is big enough, of course).
Civilopedia entry
Safe haven, shelter from storms, place to rest and refit, harbor. As soon as men went sailing, the need for harbors became evident. Early civilizations sought out natural harbors, and history has been often defined by where these maritime settlements sat. In time, breakwaters, jetties, seawalls, lighthouses, and drydocks were added, and the seabed was dredged if necessary to keep the harbor open. The harbors were the centers of trade and naval war, the launch point for exploration and end point for immigration. As technology developed, "artificial" harbors could now be built to handle modern merchant ships – such as Long Beach Harbor, created from salt marshes and tidal flats – using concrete, steel, dredge barges, and pumping stations.