Purple Mucus

Sea snails sit near the ocean floor and are rare enough that casual swimming will never build a useful stack. The workable approach is to camp the bottom with something that handles your breath, a diving helmet at minimum, and let the spawn rate do the work while you kill everything that appears.

A Solar Eclipse helps more than it sounds like it should. Eclipse enemies cannot spawn in water and mostly end up on the beach or in caves under the seabed, so what you actually get down there is the normal ocean population at a much higher spawn rate. That is exactly what you want.

The real-world parallel is deliberate. Ancient Mediterranean purple came from a sea snail and cost roughly its weight in silver, which is a fair explanation for why the game makes you work this hard for one color.