That is because team aqua got in. enter, go to the second floor, and talk to the guy. you'll battle a grunt, then be able to deliver the Devon goods.
In the Slateport museum.
he is in the laboratory in slateport that has many boats and near the bazaar
he is in the slateport museum
Mr.Briney isn't in Slateport. You can find him at the house to the left of Petalburg. (If you're looking for the scientist guy,he's in the museum.)
He's on Slateport City's museum, second floor.
Talk to the remaining aqua member in the museum in slateport city.
Oceanic Museum in Slateport City
he's not in there! he's in slasport city his worker is in the abandoned ship and he asks you to find the generator or something i forgot then you have to find it and then you go talk to him and he will tell you to go to captain stern and then go to slaport city and then he will trade you for something. hope i helped :)
A team aqua grunt gives it to you in the slateport museum after you defeat them all.
Capt. Stern is in the Museum where all the the Aqua/Magma members are conjugated around.
go past the pokemon center get in the middle of the road and go right
Captain Stern is on the top floor of the museum in Slateport City.