Great Marsh — Pokémon Definitive Platinum
Every wild Pokémon here, by encounter method.
Great Marsh lists 14 Pokémon across 5 encounter methods (grass, surf, old rod, good rod, and more), between level 10 and 55. 2 of them are on no other route's wild tables in this guide: Parasect, Swalot.
Great Marsh in the official games
The original game's table for this area:
Opens up at 4 badges — Cobble (Maylene, Veilstone City).