Defences also depend upon playing style, comfort level and other factors. In my current Game of the Month, I'm at about 1700 AD with Electronics & Tactics as my recent technologies. I have a rifle unit in most of my fringe cities, about three spies & two cavalry units nation wide. About a third of my cites are actually empty of units. Two destroyers are out searching the waves.
Officially I'm at war with the Sioux, but I have yet to fire a shot in their direction.
Barbarians landed on the east coast of my continent, and my rifle brigade smacked all (both) of them -- no other civ is nearby, so I'm rather comfortable with my situation.
Once you have control of your continent, such that there is little empty space for spawning barbarians, then the only fear is a single boat's worth of invaders. Most of the coastal cites should have a defender and the nation should have the capability of responding to an invasion situation -- in my case above, funds availabe can bribe and/or rush build units if the emergency calls.