The first step a wise ruler undertakes when willing to conquer the seas is to prepare the necessary infrastructure. Wharves, docks, warehouses, able sailors...all this helps to boost not only sea-related undertakings, but also general production capabilities of harbor cities.
This is a great policy to start the tree with, especially if you have many coastal cities. +3 Production in the Medieval Era could be a serious boost. Even in later eras, any newly founded coastal city will greatly benefit from this bonus (which is applied both retroactively to old cities, and to each new city, as long as they are found on the coast).
Maritime Infrastructure is the term used to describe the collective installations and services that facilitate seaborne mercantile activity. These include the docks, wharves, warehouses and such as well as the stevedores, shipwrights, harbor pilots, and others that handle the docking and loading/unloading of cargo. It has been estimated that throughout history one in every six jobs has been related directly or indirectly to maritime commerce. For those nations seeking wealth and expansion via the oceans, a sound maritime infrastructure is vital. At the height of the British Empire it is estimated that the crown spent almost a third of its revenue in expanding and improving the ports around the globe that serviced the empire's vast merchant fleet.