Squid

Squid are 8-armed creatures that are always passive towards the player, akin to the other passive mobs found on land. They can be found in water of any depth in any biome and spawn on any difficulty. Squid are the first passive water mob to appear in Minecraft.

Uses
When killed, each squid will drop 0 - 3 Ink Sacs that can be used to dye wool. The ink sac, like a unit of dye, can be applied directly to sheep before shearing to make black Wool or mixed with varying proportions of Bone Meal beforehand to yield light gray and gray dyes (that can also be used on sheep before shearing). Ink sacs are the only way of crafting black wool; the quicker alternative is searching for a black sheep with shears.

Alternatively, ink sacs and the gray dyes can be crafted with a wool block for a single piece of black, gray, or light gray wool.

Behavior
When moving about, the squid's tentacles open and close, giving them an appearance of pushing themselves forward. Squid will hover in place and wander around one layer aimlessly and do not deliberately interact with the player, even if attacked.

Squids become immobile when beached on the land. They can take fall damage and burn like any land mob, but they cannot drown, being aquatic creatures, yet they will stay alive if taken out of water.

Squid behave like other mobs and cannot despawn as of Beta 1.7.

History
Squid were introduced in the Beta 1.2 update. Back then, using a bucket on a squid when it was not touching another block would cause the bucket to fill with milk (this was removed in 1.4).

As of Beta 1.3, squid were given the ability to move in three dimensions, rather than only along the bottom of the body of water. An easy way to find them was to press F3, which displayed a number over their head clearly visible from above ground.

As of Beta 1.5, squid are incapable of swimming upward. When they do manage to swim upward, they drift down with a zero net gain in altitude, resulting in a jittery animation.

As of Beta 1.8 Pre-release 1, squids are the only mob that don't drop an Experience Orb when killed by the player. This has been since fixed after Pre-Release 2.

Bugs

 * A squid can end up above water, swimming through the air. (This can be done by removing the water around them with a bucket. It can occur naturally as well, but it is very rare.)
 * Despite not having feet, squid make footstep sounds when swimming across blocks.
 * In SMP, when you log in on a server with a large amount of lag, you may see flying squid above the clouds. However, they will disappear later.
 * In multiplayer, when the server is experiencing major lag, when you push a beached squid, it will appear to be "flying".
 * In multiplayer, right before the point that lag reaches levels at which every player gets booted, squid will literally glide out of the water and begin "swimming" around in the air. This is often attributable to the WorldEdit plugin, when a major action has been performed using it.

Trivia
ability to spawn in any light level and in any depth of water, source block or not.
 * Hitting a squid underwater causes them to rise approximately 1/3 to 1/2 a block.
 * Squid cannot be damaged by attacking their tentacles (unlike Ghasts).
 * Before 1.9, Squid cannot spawn in the Nether without the aid of inventory hacks, as there is no water in the Nether. As of pre-release 4 they can spawn in water created by Ice in the Nether, and will spawn very frequently due to there being no where else to spawn.
 * Real squid have eight arms and two tentacles, while Minecraft squid just have eight arms, making them more akin to octopuses. The origin of this mistake could come from the fact that squid and octopus have the same name in Swedish (Bläckfisk), which may have confused Jeb.
 * Real squids are part of the Mollusca animal phylum.
 * An easy way to find squid is to stare into a large body of water and rapidly hit the fog distance key (default 'F'). (No longer works after 1.8 update, you need to go to video settings to set the fog distance; you can still do this, but you have to hold F3 button while doing this)
 * Squid behave like some land mobs and attempt to ride a Minecart. However this is not normally very useful as rails cannot be placed underwater.
 * You can put it in and drain the water, therefore the squid will not die, and is now transportable.
 * Squid are the first passive mob that spawns at any light level, in water, and without grass.
 * When the world is loaded, all squid are aligned with the tentacles facing east.
 * It is possible to punch a squid onto land.
 * If squid are hacked into a monster spawner, they can spawn on land.
 * It is possible for squid to spawn in underground lakes or waterfalls due to their
 * Squids are the only Passive mobs that will not breed.
 * Unlike other mobs, squids can't enter "love mode".