‘Mired in Muck’ is a brief side quest in Zelda: Tears of the Kingdom in which you have to help an NPC, Bazz, by helping him get out of the sludge he is trapped under.
You will learn during the course of the game that Zora’s Domain is suddenly plagued by nasty sludge. As such, the Captain of the Guard at Zora’s Domain sets out to investigate it. While doing so, he becomes a victim of the sludge himself which blocks his gills.
Link will save him by washing away the sludge. In return, you will get a Zora Spear. So, let us look for the location of the ‘Mired in Muck’ side quest and how Link can help Bazz to complete it.
How to start Mired in Muck in Zelda: TotK
To start the Mired in Muck side quest, you have to look for Bazz. You can find him lying in front of the stairs of the Upland Zorana Skyview Tower in Zora’s Domain, northwest of the Veiled Falls.
You can also get to Bazz by fast traveling to the Joniu Shrine and then traveling northwest till you reach the coordinates 2842, 0578, and 0376.

How to complete Mired in Muck in Zelda: Tears of the Kingdom
Once you reach the entrance of the Upland Zorana Skyview Tower, you will see Bazz crying for help. He will be trapped under the nasty sludge and won’t be breathing properly as his gills are covered.
Bazz will tell you that the only way to help him is to wash away the sludge with some clean water.
There are two ways to wash off the sludge. The first one is to take a Hydrant out from your inventory and use it on Bazz. The other one is to make use of the Splash Fruit. You can pick them off the plant nearby behind a large rock.
You can use an arrow to shoot the Splash Fruit at Bazz. This will clear away the sludge letting Bazz breathe freely. Now you can go talk to Bazz who will thank you and give you a Zora Spear in return.