From Bulbapedia, the community-driven Pokémon encyclopedia.
Rub-a-Dub River is a friend area in Pokémon Mystery Dungeon. It is obtained either by buying it from Wigglytuff in Pokémon Square for 500 Poké, or if Totodile is the player character or the player's partner. It is the only area in the river area, and has a capacity of seven Pokémon.
Rub-a-Dub River is a river that has three paths to it. It looks like it is used for irrigation, as there is a water wheel attached to some sort of building. The building has four waterfalls coming down the sides of them. At the center of the river is a small island with a tree.
Pokémon that love to frolic in water gather at this beautiful river where pure water flows.