Answer 2:
In general, things float in water if the
density (mass per volume) of the object is less
than that of water. Since ships are basically
hollow, they are, on average less dense than water
and will float.
The next question is: how much of the
ship will be above the surface of the water?
You can calculate the percentage of the ship that
is underwater by comparing the weight of the ship
to the weight of the water that is displaced by
the volume of the ship that is under the surface.
If you take a ship that has a mass of
1,000,000 kilograms and lower it into the ocean 
it is going to sink into the water until it has
displaced 1,000,000 kilograms of water or, since
water has a density of 1000 kg /m^{3},
until 1000 cubic meters of ship volume is under
water.
Hopefully, the total volume of this ship is more
than 1000 cubic meters or the ship is going to be
in trouble.If you add more weight to the ship,
the ship is going to float lower in the water.
Obviously it is a bad idea for the top of the
ship to be too close to the surface of the water
because if the hollow parts of the ship start
filling with water, it will become more dense on
average and eventually sink.
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