Instructions
Make your way to Pastoria City.
Enter the Great Marsh. It is near the town entrance when coming from Route 213.
Pay to play the safari game. You will be given 30 pokeballs to use, but if you travel more than 500 steps, the safari game ends.
Go to a pond within the Great Marsh and use Surf to travel into the pond. Keep moving around and getting in random battles until you find a Marill. Marills can be found anywhere in the Great Marsh, but are more likely to appear in water while using Surf.
Catch the Marill with a Safari ball.