Shark Fin

Sharks are the bottleneck, not the drop. Ocean water holds only a few at a time and you spend most of the trip swimming back up for air. Two things fix that. A Blood Moon floods the Ocean with them and pays out rare dyes at the same time, and a sealed pocket of air, or a Gills Potion, lets you sit at depth and let the spawns come to you. A Water Candle and a Battle Potion do the rest.

Once the world turns, stop swimming. Sand Sharks surface in the Desert during a Sandstorm, so you can farm fins on dry ground with no breath meter to manage. The odds per kill are worse than an Ocean Shark, but the kill rate is so much higher that it is not close. A Shark Statue on a timer covers you if you need an exact count, and shimmering the right potions can shake a few loose too.

Fins taken from Sand Sharks come out tinted, and the tint follows the same rule as Gel: the first one into a stack decides the color of everything you pile on afterward. Merge two colored stacks and the whole thing takes the color of the one you dropped on top. The effect is cosmetic and fragile. Item frames show the plain sprite, and dropping a stack or rejoining the world washes it back to normal, so it is a curiosity rather than something to plan around.