The S.S. Liki is one of the two ferries that you can ride across Spira. This one ferries people between Besaid Island and Kilika Island.
The team boards and rides the S.S. Liki fairly early on in the game. Refer to the walkthrough and strategy section for details:
You can return to either Kilika or Besaid later on in the game using the Airship and ride the S.S. Liki again between the two locations.
