Did you see the museum with all the Aqua Grunts blocking the entrance? South of that is a large brown building with a sign next to it. Go in and Cap. Stern is the one at the table looking at the blueprints for a ship. Talk to him, then you can enter the museum. Before you move on and fight the grunts, talk to the Aqua Grunt right north of the exit of the museum. He will give you TM46 Thief. Hope I helped.
you go into the aquarium place, and he will be there.
in the oceanic museum
You go to Slateport and go to the museem and you can find him on the second floor
a guy will ask u if u can deliver Devon goods to captain stern..captain stern is found in the museum of slateport(specifically 2nd floor).
In Pokémon Alpha Sapphire, after you reach Lilycove City, Captain Stern can be found at the Stern's Shipyard located in Slateport City. He is not in Lilycove; instead, players must return to Slateport to continue the storyline involving Team Aqua or Team Magma. The shipyard is where you will encounter him after the events involving the Team's plans.
You deliver Devon's Goods to Captain Stern who is in Slateport City's museum.
Head to slateport city and go to the SS tidal captain stern will be nearby.
You have to talk to the scientist guy at the ship place in slateport city
In Slateport city harbour you will see capt. stern being interviewed with a large crowd talk to capt. stern and you will go into the harbour thing and then go to their hideout in lilycove city.
he will be at slatport city after you find the team magmas hideout having a interview
Go back to Slateport town. Inside the ferry building, you will find captain stern and the submarine being robbed by Team Aqua.
He's in the Slateport museum but you might have to go talk to a guy in the big, brown building (go up from the entrance and he's in front of some blueprints).