Cannot move ship if six are built in a circle
After I built six ships around the same hex with only one settlement connected to them as part of this circle (don't ask me why I did it), I was unable to move any of those ships. However, according to the official rules, it shouldn't count as a closed shipping route until it connects to a second settlement/city/metropolis (http://www.catan.com/files/downloads/seafarers_rv_rules_111105.pdf p.4).
I don't think that it's clear that you're reading the rules in the way the designer intended. Sure, if you insist on a literal interpretation you're right. But that leads to a board state that doesn't show what ship is free to be moved. Which probably isn't the intended consequence of this rule.
I posted this question to boardgamegeek:
Usually, Catan representatives are pretty quick at providing official rules clarifications there.
And as expected a Catan representative quickly chimed in on BoardGameGeek and pointed me towards this FAQ entry:
A circle of six ships should be considered a closed shipping route. So you shouldn't be able to move them.