The Great Marsh in Pastoria has undergone a fair few changes since Diamond & Pearl. Firstly, the marsh area isnt filled with wild grass, merely outlined with it. A number of Pokémon are found in the Great Marsh and only in the Great Marsh so you will have to keep coming back if you want a specific Pokémon. Each day, there are specific rare Pokémon that are seen only by looking out along the Marsh with the binoculars upstairs. There you will see the Pokémon and the area they are in. There are six areas in the Great Marsh, and you can walk between them or have quick access on the internal train.