It comes after fishing few minutes, look at the screenshot for the spot.

The item is a part of a collectible items used in a "secret quest challenge". This one been used in the second episode (The Tale of the Fallen Kingdom II - August 2020) :
https://blackdesert.pearlabyss.com/ASIA/en-us/News/Notice/Detail?_boardNo=1752#_pageNo=1,_sortType=2