Instructions
Travel to "Hearthome City" and talk to the Fisherman near the city's entrance to receive the "Good Rod." The Good Rod allows you to capture better Pokemon in any body of water throughout the "Sinnoh" region. Head to "Veilstone City" after talking to the fisherman.
Walk south from Veilstone City to enter Route 214. Traverse through Route 214, battling trainers and wild Pokemon encountered along the way. Head south to reach the "Hotel Grand Resort"; walk through the main hall to reach Route 213.
Head westward through Route 213 and enter "Pastoria City" after defeating the trainers and wild Pokemon along the way. Continue walking west and turn north to reach a large building, the Pastoria City Great Marsh; enter the building and walk toward the counter on the west side.
Talk to the receptionist to pay a 500 Pokedollar fee for 30 Safari Balls and 500 steps. The Great Marsh, and other "Safari Zones" in Pokemon games, limit the number of steps you take within the area. Take the train to the Great Marsh from the main lobby.
Head to any area body of water within the Great Marsh to find Barboach. Use the Good Rod from the edge of the water to reel in Barboach; find Barboach with the Good Rod from the back of your Pokemon using the HM, or Hidden Move, "Surf" to travel in the water.
Throw your Safari Balls at Barboach to capture it. Great Marsh Pokemon battles give you the option of throwing a Safari Ball, mud or bait. Throwing mud increases the chance of capturing a Pokemon, making it more likely to run; throwing bait makes a Pokemon less likely to run and harder to capture. Keep throwing Safari Balls until Barboach stays inside.