From Bulbapedia, the community-driven Pokémon encyclopedia.
Stern's Shipyard (Japanese: クスノキ ぞうせんじょ Kusunoki Shipyard) is a building owned by Captain Stern, featured in Pokémon Ruby, Sapphire, and Emerald. Stern's Shipyard is a large brown building located next to the ocean. In front of the building are two houses, and around the building are three small wooden boats. It is located in the eastern part of Slateport City, and is where the S.S. Tidal was constructed. The Shipyard also takes old boats and sinks them, to make habitats for the Pokémon who live around the city and its neighboring beaches.
When the player first arrives in Slateport the Oceanic Museum will be inaccessible. The player must first talk to Dock at the 1st floor of the Shipyard, and then head to the Museum to meet Capt. Stern.
After the player obtains five Gym Badges, Mr. Briney comes to Slateport City to help out with the construction of the ferry. Then, after the player beats the Elite Four, the ferry will take them to the Battle TowerRS or to the Battle FrontierE.