Are you able to build a settlement from a separate ship?
Boats are like roads with a few exceptions:


they must border at least on one side to water.
they require wood and sheep to build.
you can pivot the last ship in a row of ships (but it needs to stay connected).
they are affected by pirates


You can only change from road to ship (or ship to road) through a settlement (or city). 

In order to build a settlement on an Island where you have no settlement yet, you need to build a row of ships starting from a settlement at the first island until you get to the second island. You can now continue building ships along the coast, or you can build a settlement. And if you are the first to build on an island, some scenario's reward you with an extra victorypoint.