Take a look at the following setup:
Given this board state, can blue move any of his ships? He was previously unable to, then white built the circled settlement, breaking blue's trade route.
The rules in seafarers say that a ship can be moved only if it doesn't connect two cities and it is not in the middle of a line of ships. Since a settlement interrupts a longest road, does it also count as breaking the above sea route into two disjoint lines of ships?
(NB Blue's settlement on the right was an original, the blue settlement on the left was placed after being reached with the shown sea route.)