Pufferfish

The old Pufferfish texture from early 1.7 snapshots.

Pufferfish is a food and brewing item in Minecraft. It was added in the 1.7 update. However, as a food item, Pufferfish is actually negative in terms of hunger restoration, removing 5 hunger points (directly, not counting hunger used to heal from the poison damage) due to effects it inflicts, so it is best saved for brewing or for taming Ocelots.

Food

When eaten, Pufferfish will restore 0.5 Hunger, but also give the player the Hunger I effect for 15 seconds, causing them to lose 3 Hunger Bars (as much as a Golden Carrot or some cooked meats would give them). That's not all, though: It also gives Poison IV for a full minute and Nausea I for 15 seconds, causing the player to remain at 0.5 health for a full 48 seconds unless they eat something immediately afterwards.

Taming

Pufferfish can be used like any other fish to tame Ocelots. Just find a space at least 7x7, stay still, wait for the ocelot to "sneak" up to you, and keep right-clicking it. Note that Ocelots only spawn in Jungle Biomes.

Brewing

When brewed into an Awkward Potion, Pufferfish makes a Potion of Water Breathing. This is the only way to get Water Breathing in legitimate survival mode. 

Trivia

• Pufferfish, Golden Carrots and Spider Eyes are the only brewing ingredients that can be eaten.
• The aforementioned items and Poisonous Potatoes are the only food items that are (can be in the sake of the potato) poisonous.
• This is the only way to legitimately get the Nausea effect in survival mode.