see, now that I know it's possible to build structures like that on a barge, I wonder how hard it would really be to found a nation on such barges.

Think about it, you purchase one or two decently sized barges, fill them up with interested people and enough resources to get things started and move out to international sea lanes. Over time you add more barges and decommissioned warships for defense as your little communities grows while enjoying the fruits of trade with passing ships.

Hell, if you plan things properly, you could even move around to avoid bad weather, potentially keeping hurricanes from being an issue...