The Cargo Ship is a boat that takes you from Junon by sea across to Costa del Sol and back again if you choose to return.
The story takes your party in to the Cargo Ship when they leave Junon in pursuit of Sephiroth. The ship arrives in Costa del Sol just before the team makes their way to Mount Corel. You have the option of using the Cargo Ship to return back to Junon if you’d like by returning to Costa del Sol and speaking to the sailor standing in front of the ship.
Check out the Cargo Ship walkthrough page for additional details.
There are a number of items that you can pick up while onboard the ship. You cannot return to the Cargo Ship later on in the game so make sure that you grab them while you’re there:
The map below shows the two locations where the ship will dock: