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Captain Stern is on the top floor of the museum in Slateport City.
Capt. Stern is not in Pokemon Leaf Green. He is only in Pokemon Ruby, Saphire or Emerald.
You will need to give the Scanner to Captain Stern in Slateport City. Once you obtain the Clear Bell from him, return to the Abandoned Ship/Sea Mauville and you can encounter Ho-oh at the north-western edge of the ship.
In the slateport city boat dock place
While in Lilycove City, you can find Captain Stern in that museum place where Team Magma/Aqua are standing by. I forgot how to get rid of them, but once you do, you can find Captain Stern in that museum place I mentioned.
You go to slateport and you deliver the Devon goods to captain stern.
You get it from Captain Stern in Slateport City after finding the Scanner in the Abandoned Ship but you need a Pokemon with Dive.
You deliver Devon's Goods to Captain Stern who is in Slateport City's museum.
You can either go to the next route or get the exp.share from the president.
He is in the ocean museum in slateport city on the second floor in the northeast back corner wearing a brown coat
It's the Seafloor Cavern... An underwater cave where the legendary pokemon, Kyogre is sleeping.
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)