Blood Feeder

Crimson water is a harmless puddle right up until the world enters hardmode. After that, any pool down there can hold these. Nothing summons them and no event is involved: be in crimson water once hardmode has started and they spawn on their own. They come in numbers, and they hit far harder than the corrupted fish you were shrugging off in the same water an hour earlier.

This is a positioning problem more than a combat one. Being in the water with them is the worst place to be, since your movement suffers and theirs does not. Drain the pool, fight from the lip of it, or bring something that reaches underwater. A casual dig for hardmode ore in the crimson is the usual way people find out they exist.

Nothing about them rewards farming. Monster Meat is the only unusual thing they carry and it turns up rarely enough that you will pick one up incidentally long before a dedicated pool run pays for itself.