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).
after you beat the elite for the ferry is complete and captain stern is the captain of the ferry (i think check to find out)
you go into the aquarium place, and he will be there.
Head to slateport city and go to the SS tidal captain stern will be nearby.
You deliver Devon's Goods to Captain Stern who is in Slateport City's museum.
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.
Captain Stern is on the top floor of the museum in Slateport City.
Go back to Slateport town. Inside the ferry building, you will find captain stern and the submarine being robbed by Team Aqua.
first, go to the shipyard. Captain stern isn't here though. Talk to the man that you first come across that seems to be staring at a piece of paper. He'll say that he is called Dock and then tell you that Captain stern is at the museum. Finally, you know where he is! Go to the museum at the east of town, then talk to the man on the second floor. He is Captain Stern. He'll take the goods,thank you,and arrange to leave.Before he can move,2 members of team aqua will come in and try to steal the goods.Battle them,and when you defeat them both,the aqua leader will come will come,give you a brief warning and leave. .
Go to the shipyard, and talk to the guy standing by the table with a map on it. He will tell you Captain Stern isn't there. Leave the shipyard, and go into the museum. Go to the backroom, and talk to the scientist. He is Captain Stern. Team Aqua will come inside, and be prepared for two battles. After you beat them, you can give the parts to Capt. Stern. Hope this helps! :)
he will be at slatport city after you find the team magmas hideout having a interview
At Staleport City
The captain went to the stern of the ship to see if anyone was following them.