Water

Water is a fluid that fills the sea and flows out from a source.

Obtaining

Water fills the open space below y=64, which makes the sea. It also forms lakes, and springs in cave walls.

An empty bucket takes a water source, and a water bucket places one.

Ice becomes a water source when it melts, and when a player breaks it above a solid or liquid block.

Water is needed by:

Farmland, which stays wet with water within 4 blocks across, level with it or one above;
Sugar cane, which is placed only on grass or dirt with water beside that block;
fishing, which gets bites only with the bobber in water;
Squid, which spawn in water.

Water flows 7 blocks from a source on the flat, spreading one block every 5 ticks.

Flowing water between two sources, over a solid block, becomes a source.

Flowing water breaks the non-solid blocks it reaches, such as plants and torches, into their drops. See Fluid#What stops flow.

Lava with water beside it or above it hardens into obsidian or cobblestone.

Still water sources freeze into ice in snowy biomes.

An entity in water:

is pushed the way the water flows;
stops burning, and does not catch fire;
takes no fall damage from a fall that ends in it;
drowns with its head under, unless it is a squid: 2 damage 16 seconds after going under, and 2 more every second after that.

A player with their head underwater mines five times slower.

No explosion breaks water, and one centred in water breaks no blocks at all.