how much water does it take to float something?

Yes, a form fitting pool 1 mm wider than a large boat would be able to float it. It is about how much water is displaced, and the displaced water need not remain in the pool; and need not even need to have been there to start with.

What matters is the height of the water column, if it reaches the line of flotation, then the ship is floating.

Yes. Vincent is right.

A floating object displaces its own weight of water. So in theory you could float a 20,000 tonne ship in a 20,000 cubic metre pool. 20,000 tonnes of water would flood over the side of the pool and run away, leaving only the thimblefull as a thin layer between the pool sides and the ship up to the plimsoll line.

Or just think about sailing the ship into a narrow canal lock and shutting the gate behind it.

All your doubts will be cleared , if you read about Panama Canal Locks.

http://en.wikipedia.org/wiki/Panama_Canal_Locks

See the pictures and think why they have left out enormous space surrounding the ship.

For floating there must be sufficient water surrounding the ship.Further think of the mass of the ship and think what will happen if it hits the shore accidentally .

Consider a horizontal plane of water below the ship and the depth of water below this plane is small.

If you remove the ship from the water , all water surrounding it should fill up the space created by lifting the ship out side.

( Note water from below the horizontal plane already we refereed, could not help in filling up this space.)

The mass of water that fill up this must equal or greater than the weight of the ship.

Then alone it can float in water if the ship is replaced again.

Now think whether 1mm thickness of water surrounding the ship will be enough to fill up the huge space created by lifting the ship up out of water

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http://en.wikipedia.org/wiki/File:Panama_Canal_Shi...

Additional details:

The above answer is assuming that the floor is a few mm below the bottom of the ship.

No, it has to be able to displace more pounds of water than the ship's weight. If there is not enough water to displace, it won't float. So, the pool has to be deep enough and wide enough to allow the ship to displace more than its weight in water.

Some of these other posters don't seem to be very bright. If the water ran out of the pool as the ship went in, the ship wouldn't be constantly displacing that amount anymore, would it? It would be displacing the amount LEFT IN THE POOL, which would be considerably less than the weight of the ship in the Asker's example.

It is not the amount of water. It has to do with the mass of the object. The more the mass the more the water to support it. The less the mass.... well vice versa

It all has to do with the weight and surface area of the objects placed in the water