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Naval Infrastructure is an Economic Policy Card in Civilization VI.
Strategy[]
Naval Infrastructure is well-suited for maritime empires with many Harbors. It's generally not worth slotting just to get some extra 
Gold from adjacency bonuses, but after discovering Mass Production and unlocking Shipyards, Naval Infrastructure becomes five times better (since Shipyards provide 
Production, which is worth roughly four times as much as Gold, equal to a Harbor's adjacency bonus). It's fairly easy to get a +3 adjacency bonus on a Harbor by placing it next to a City Center and either another district or a water-based resource, which translates to a respectable +3 Gold and +3 Production for each coastal city with a Shipyard.
Naval Infrastructure also boosts the Golden Age dedication Free Inquiry, which causes Harbors to produce 
Science equal to their adjacency bonus.
Civilopedia entry[]
If a civilization was going to have a naval and merchant fleet, it needed a lot of other things – docks, slips, wharves, warehouses, breakwaters, lighthouses, not to mention all those stevedores, shipwrights, harbor pilots, and others – to float ships and keep them afloat. Without this naval infrastructure, navies don’t last long. In the Middle Ages, for the first time, some kingdoms devoted substantial planning and funds to their naval facilities. For instance, the threat of Ottoman expansion brought the Venetian Republic to establish extensive shipyards (including the famous Arsenal), while the Fatimid Caliphate and Byzantine Empire needed efficient naval infrastructures to keep killing each other for two centuries.
