- Divine Beast Vah Ruta main quest completed
The idea here is to follow and protect the letter container until it gets to the end of its journey. Mostly this just involves patience.
The first time interaction is required is when the container reaches a wooden barrier. Pick it up and throw it over, or use a bomb or Bomb Arrow to destroy the barrier if the letter container is far enough away to avoid destroying it.
Now move ahead of the letter carrier slightly and throw a bomb into the water a bit ahead of it. Be very sure the bomb is not close to the carrier or it will destroy it. Follow bomb and carrier down the river, detonating the bomb when it gets close to enemies or barriers and throwing another to replace as needed. There should be only one more barrier, and enemies mainly tend to ignore the container and focus on you instead.
If you wander too far from the container, it seems not to move very much when it is out of sight. Keep it in sight to keep it moving. As long as you remember where you last saw it, chances are you can find it again since it does not move very much when it is not in sight. If you lose it completely, return to Finley to start over.
Try to be aware when the next Blood Moon is if you begin the quest in the evening. The Blood Moon cut-scene will cause the quest to start from the beginning.
When the container finally reaches the end of its journey at the small inlet on Mercay Island, approach to trigger a cutscene.
Set the quest as the active marker (if it is not already) and return to Zora's Domain to head to the marker and complete the quest. The reward is a gold rupee.
Follow the letter down the river without losing sight of it, and find out who receives it. The letter's casing is fragile, so take care when pursuing it downstream.
If you lose sight of the letter, return to Finley to try again.
It turns out that Sasan picked up the message. He's heading to Zora's Domain to meet Finley now.
If you want to see what happens, you should get to Zora's Domain too...
Sasan arrived safely at Zora's Domain and met with Finley. Your part in the matter is concluded, though it remains to be seen whether Sasan can put on the muscle Finley hopes for.