Black Ink

Squids only appear in ocean water, and they hang near the bottom rather than at the surface. That means the real obstacle is your breath meter, not the fight. A Diving Helmet or Neptune's Shell turns a farm that takes twenty short dives into one long swim.

A Solar Eclipse pushes spawn rates almost as high as a Blood Moon, and the enemies it brings cannot spawn in water. What you get is a crowded ocean full of ordinary sea life while the eclipse monsters mill about on the beach behind you. Occasionally one wanders in, so keep a weapon out, but for the most part it is free ink.

Nothing but Black Dye. If you are not dyeing anything, ink is vendor fodder, so only farm it when you have a specific outfit in mind.