Another addition for the Black Pearl (because Shadow didn't know the name of the sea nymphs anymore):
It's in the Kelp Forest, located on the map with the entrance to the sea nymphs' cave.
From the entrance of the nymphs' cave, swim northwest into the left main part of the map.
Swim north until you see a small cave with a big fish lurking in it (on the right side), switch into Fish form and swim past the big fish.
Switch to Sun form and follow the small passage behind the fish (avoid the thorns) and collect the Black Pearl at the end of the passage.
Also, Naija's thoughts for several treasures and their effects:
Big Seed:
"I discovered a giant seed in my sanctuary.
When I brought it home and planted it, it sprouted into a colorful garden."
(Exact effect on Naija's cave still unknown)
Anemone Seed:
"When planted, this seed sprouted a garden of anemones in my cave."
- It spawns 3 anemones below the kitchen.
Sun Key:
"A strange shiny object... I had no idea what it was for, but I decided to keep it."
- Spawns 2 mechanic fishes in the "sun temple like" part of Naija's cave.
Black Pearl:
- Spawns a clam you can sleep in in Naija's garden.
Walker baby:
- Spawns an adult walker in Naija's garden.
