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.)
mr briney is at slateport helping the people make the ship
in slateport helping the ship people
dewford and slateport
you sail with mr briney, to dewford, deliver the letter and beet the gym, then sail to slateport.
Use the boat with mr. briney to the left of petalburg to get to dewford. Then, after you deliver the package for the devon people in the cave, you can boat again to slateport.
Captain Stern is first located at Slateport City. After saving Mr Briney's Peeko on the tunnel, go to Briney's hut and talk to him. After delivering the goods at Steven and finishing the gym at the Dewford town, talk to Mr. Briney and go to Slateport City. Go up and enter the museum. On the second floor, captain stern was standing next to the scale model of SS Anne.
if you haven't already delivered the letter to steven then go to slateport city by talking to mr briney on the pier if not then go to the cave beind the trees and go in and get to steven and talk to him then go to mr briney
You need to use mr briney's ship to reach slateport city then go north to reach mauville city.
Mr briney is in a house near petalburg woods.
Answer:Mr. Briney is first seen near the Rusturf Tunnel, asking the player to recover his Wingull, Peeko, from a Team Aqua or Team Magma Grunt. Once Peeko is rescued and returned to Mr. Briney, he will offer to give the protagonist rides in his boat to Dewford Town and Slateport City until he/she gains the badge from his/her father. He later becomes the captain of the S.S. Tidal. His house is located on Route 104.
The 4th gym is in mauville city to get there you need to use Mr Briney's ship to dock at slateport city then you need to go north until you reach mauville city.
This is on Pokemon Emerald but I think It's the same. After you help Mr. Briney save Peeko, you'll be able to hitch a ride on his ship to Dewford town and Slateport city. His house is located on route 104 east of Petalburg city.