Spring of the First Dewdrop is a Genshin Impact local specialty that can only be found in Fontaine's underwater Morte Region. It's used as an Ascension material for Navia and possibly other characters in the future.
Whether or not you've pulled for a character who needs Spring of the First Dewdrop, I highly recommend collecting it in advance so you're never in short supply.
Where to farm Spring of the First Dewdrop in Genshin Impact
Spring of the First Dewdrop materials are clustered in the Morte Region, an underwater area close to the Fort Charybdis Ruins. These Dewdrops only appear underwater, so you'll have to dive to collect them. Additionally, you'll have to complete the In the Wake of Narcissus quest chain to unlock some of the areas marked above.
If there aren't enough in your World to Ascend a character in one day, I encourage you to enter another player's World through Co-op Mode but make sure to ask permission before collecting their resources.
How to farm Spring of the First Dewdrop in Genshin Impact
Spring of the First Dewdrops don't require any special methods for collection. Simply interact with them to pick them up when you spot them. However, since they blend with the background, you can use Elemental Sight to differentiate them from underwater clutter. With Elemental Sight turned on, the Dewdrops show up as easily identifiable blue blobs.