Pufferfish

Pufferfish is an edible brewing ingredient caught by fishing. They are yellow in colour with blue fins and spines.

Fishing
The most common way of obtaining pufferfish, like any other fish, is fishing. The chance of one being caught can be increased with the 'Lure' enchantment.

Guardians
Another method of obtaining them is through defeating Guardians. This method is very inefficient, as the drop rate is roughly 19% (2.5 / 13).

As 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). This makes eating pufferfish incredibly dangerous and non-beneficial.

However, pufferfish can be used to tame ocelots without the risk of getting them poisoned.

As a brewing ingredient
Brewing a pufferfish into an awkward potion gives a potion of Water Breathing, which allows players to breathe underwater. One pufferfish is enough to produce enough potions for a maximum of 24 minutes of Water Breathing. Using a pufferfish as a brewing ingredient is the only way (aside from rare drops from witches) to get a potion of Water Breathing in Survival mode.

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.