After beating the first gym, you can enter this route. It is full of marshlands and swamps, with poison, grass, or water types at every twist and turn. It is also completely possible that you will encounter a Totodile, which is rumored to have been released here by its previous trainer.

Pokémon found here:
Common:
Poliwag
Tympole
Wooper
Lotad
Ducklett
Shroomish
Uncommon:
Psyduck
Dewpider
Gulpin
Foongus
Rare:
Surskit
Toxicroak

Fishing:
Magikarp (Common)
Barboach (Common)
Goldeen (Uncommon)

Traded:
Skrelp for Pidgey

Items:

Given:
Fishing Rod

Found:
Great Ball
X Attack