In the Catan Explorers and Pirates expansion, settler ships can convert to settlements upon discovering unexplored territories. Can settler ships also convert to settlements on the starting island?
As long as you observe the usual Catan distance rule (2 paths between settlements) you can use a Settler Ship to build a settlement on any unoccupied intersection where all the adjacent hexes have been discovered.
This is set out in the rules for Scenario 1: Land ho! on page 5 of the Catan: Explorers & Pirates rulebook:
Building a Settlement with the Aid of a Settler Ship
If either end of one of your settler ships points toward the corner of a terrain hex, you may build a settlement there.
The additional note on the same page relating to building settlements upon discovering unexplored territories is just noting that Settler Ships are the only way to gain a foothold on the newly discovered territories.