Surfing
If any of your Pokemon happen to have learned Surf, you can surf around on the water. Approach the edge of the pond, press "A" and select "Yes" when prompted. You will then hop on your Pokemon and be able to ride around in the pond to encounter wild Pokemon.
Fishing
You can fish in the Celadon City pond with the Old Rod, Good Rod and Super Rod. Approach the edge of the pond and use any of these rods by selecting them from your inventory. This will lead you to a wild Pokemon encounter. The Pokemon you encounter vary depending on which rod you are using.
Grimer
When surfing on the Celadon pond, you have a 90 percent chance of encountering a Grimer. This is a Poison-type Pokemon that looks like a purple pile of sludge. Grimer's entry in the "Pokemon SoulSilver" Pokedex reads, "Wherever Grimer has passed, so many germs are left behind that no plants will ever grow again."
Muk
Muk makes up the remaining 10 percent of the Pokemon found while surfing on the Celadon pond. Muk is the evolved form of Grimer, so you can save yourself the trouble of leveling and evolving your Grimer if you happen to encounter one. Its entry in the "Pokemon SoulSilver" Pokedex reads, "Its body is made of a powerful poison. Touching it accidentally will cause a fever that requires bed rest."