The importance of the Naval yard depends on a number of factors. Most fundamentally, if you're on a watery world you'll need it far more and earlier than on a world with lots of big dry continents. From there, it depends on how much you like ships. If you're a determined landlubber, you can give this improvement a pass entirely. If you're planning a modest trade-and-explore navy, with maybe a couple of patrol vessels along your cost, then build one or two naval yards at key bases, and co-ordinate your seafaring from there. If you're planning a full-scale armada for rapid growth and conquest, you'll want a naval yard everywhere that can launch ships. ( It's worth noting, however, that while it's possible to build a naval yard at an inland base that adjoins a one-or-two hex lake, there's absolutely no reason to do so unless that lake has an outlet to the open sea ).
The whole equation can, of course, be made moot by simply
building the Maritime Control Center , which
gives you an automatic Naval Yard at every base. If you think you might eventually
need four or more Naval yards, the Maritime Control Center is the most economical
alternative.
