Rain Flower can be acquired only when it's raining, on an island surrounded by rocks. This is a necessary item to complete Rain Flower Quest (see below) and get Recipe for Boomerang.
Where to Find
From the main island, use your Raft to sail directly north where you'll see two small islands. From there, sail directly east, where you should see some rocks to the north. Those are the bottom part of a huge crescent of rocks surrounding an island.
The island appears to be inaccessible, since every time you try and get close you'll get the message "The waves are too high to move!" However, if you hug just next to the rocks and go around counter-clockwise, as pictured, you'll successfully make your way to the island.
Once you arrive, head into the grass icon and use 4x Strong Rope to scale the two cliffs to the area with blue flowers. Note that it must be raining in order for the Rain Flower to appear!
Rain Flower Quest
During the game, you'll occasionally see a bottle wash up on shore. Use the Mallet on the bottle to obtain Letter in a Bottle. In one of these letters, someone will talk about their sick relative and plead for you to send medicine. This letter will appear in a blue bottle. In order to complete the quest, you need to do the following things:
- Acquire Rain Flower (hopefully you just did ;)).
- Use Mortar on it to create Blue Medicine.
- Use Jar on Blue Medicine to create Blue Medicine in a Jar.
- Face the shore and use Blue Medicine in a Jar from the ITEM menu to throw it out to sea.
After you have thrown the medicine out to sea, the Letter description will change to, "You've read the letter. ...It seems 1 Blue Medicine in a Jar are needed. ...Hope they have arrived safely."
After a day or so, you should get another bottle on shore. Smash it with the Mallet and there will be a Recipe for Boomerang inside! In order to use it, go to the items menu, and tap on it to use it.