The Salvage Ship is an Al Bhed vessel that Tidus finds himself on after being rescued by Rikku from the Ruins. This is fairly early in the story and you cannot revisit the Salvage Ship later on in the game.
As mentioned above, the Salvage Ship is a location that you briefly visit after being saved by Rikku and her Al Bhed companions. There are also a few scenes that take place on the Salvage Ship after Rikku and Tidus’ mission in the Underwater Ruins. Refer to the early portions of the walkthrough for more information: