Another Economic Policy which depends on adjacency bonuses, Naval Infrastructure is not that important unless you're building a trade empire and have many well-situated Harbors.
Don't forget that the adjacency bonus for the Harbor district is Gold! So, this policy will potentially increase the size of your treasury. Again, it is only worth using an Economic Slot on it if you are either focusing on Gold and will use it to purchase lots of stuff, or are so desperate for Gold that you need every extra bit. However, keep in mind that another Economic Policy, Caravansaries, has the potential to yield more Gold! As a rule of thumb, you should go with Naval Infrastructure if you have well-situated Harbors (meaning they have good adjacency bonuses) and Caravansaries if you have many Trade Routes.
Civilopedia entry Edit
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.