Besides lava, fire or a creeper invasion, nothing in "Minecraft" can ruin your perfectly constructed project like a flood. If conditions are right, water can flood over nearly unlimited blocks in the game. Unlike the significantly more dangerous lava, getting rid of too much water is usually not dangerous and shouldn't cause too much trouble even for a "Minecraft" novice.
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Block off the source of the water. Player-placed water all flows from one source block. Block off that one source block to stop the water from flowing.
Place a block onto the water's source block. This will not only stop the water from flowing, but also destroy the source block. Keep in mind that you cannot get the source block back once it's been destroyed.
Craft a bucket by placing three iron ingots in a "v" pattern on your crafting table. Right-click a water source block while holding a bucket. This will pick up the source block. By removing a water source block, you will remove all the water flowing from it.
Block off an area of water that you want to drain, and then either pick up every block of water with a bucket or destroy each source block by placing a block on top of it. This is necessary to remove still water.