According to a clinical study on the patients, which is due to be published in the journal Clinical Toxicology, the men started seeing and hearing things after contracting a rare form of hallucinogenic poisoning from the Salema fish they were dining on.
The species is a popular food fish and is not normally hallucinogenic.
Ichthyoallyeinotoxism, or hallucinogenic fish poisoning, is caused by eating the heads or body parts of certain species of herbivorous fish and has previously only been recorded from the Indo Pacific.
The poisoning can start to cause vivid hallucinations within minutes of eating a poisonous fish and may last for days, often with no other effects. There is no antidote.
The effects of eating ichthyoallyeinotoxic fishes, such as certain mullet, goatfish, tangs, damsels and rabbitfish, are believed to be similar to LSD, and may include vivid and terrifying auditory and visual hallucinations. This has given rise to the collective common name for ichthyoallyeinotoxic fishes of “dream fish”…
(Found by Bill Gibson, who also gave me the post title – thanks)