Pufferfish

Pufferfish is an edible brewing ingredient caught by fishing.

Fishing
Pufferfish can be obtained by fishing. The chance of one being caught can be increased with the Lure enchantment.

Guardians
Another method of obtaining them is through killing guardians and elder guardians. This method is very inefficient, as the drop rate is roughly 0.325% (2.5% probability of a rare drop times 13.0% probability that rare drop will be a pufferfish).

Food
Eating a pufferfish restores and 0.2 saturation points. It also gives the player III for 15 seconds (total loss ),  II for 15 seconds, and  IV for a minute, forcing the player down to  for 48 seconds (30 seconds on peaceful).

Ocelots
Pufferfish can be used to tame and breed ocelots (except in pocket edition). Unlike players, ocelots will not get poisoned.

Brewing ingredient
Brewing a pufferfish into an awkward potion gives a potion of Water Breathing, which allows players to breathe underwater.

Trivia

 * The consumption of pufferfish inflicting the player with nausea and poison is actually based on reality, as real pufferfish contain the extremely dangerous toxin Tetrodotoxin within their organs, especially their liver. Muscle tissue and blood only contain trace amounts. In East Asia, pufferfish are considered a delicacy known as fugu, as small parts of the fish are actually edible.
 * If this dish is prepared incorrectly, it can be lethal through respiratory paralysis. Therefore, any chef preparing fugu would have to have special licensing/certification to prepare this dish.