Theres a Water section you can enter, right before you would start to explore the Fungal Forest. This require you to get the angel jump out of Tengoku, the slime transformation out of Fungal Forest, so that you can get the Mermaid scale out of Spirit City Theres a rat area at the bottom of Spirit city that has it, you can see one of the bombs being thrown by a rat when you approach the Slime Grate.
But yeah after you have the mermaid scale you can enter the water area, its to the right of the Fungal Forest warp.
and the bottle is in Witchy Woods, Same goblin map that has the apartment and the mimic.