Instructions
Head to the Great Marsh, located north when coming from Pastoria City. Enter the building and go up to the second floor.
Look through the binoculars. Each day, different Pokemon appear in the Great Marsh, and the binoculars let you see the ones you can catch today.
Go downstairs and pay $500 to enter the Great Marsh if you see Croagunk through the binoculars. You receive 30 Safari Balls.
Walk around in the grass inside the park. Eventually, Croagunk appears. You do not fight him to catch him. Throw Safari Balls at Croagunk to grab him before he flees. If he runs away, you need to find Croagunk again.