Answer 1:
I don't know how much scientific language is
really necessary to explain this phenomena.
It all comes down to a principle called
"buoyancy," and can be explained by what is known
as "Archimedes' Principle." What this principle
basically states is that a floating object
displaces an amount of liquid equal in weight to
the weight of the object. This means that the
amount of water displaced by a boat, no matter how
full that boat is, weighs as much as the boat.
The weight is related to the volume of water
displaced by Weight = (Gravity)*(Density of
Water)*(Volume of water displaced) This is
where the dimensions of the boat becomes very
important. The volume of water displaced by the
boat is equal to the volume occupied by the part
of the boat that is below the surfaces of the
water. Once the volume of displaced water is
greater than the total volume of the boat,it means
that the top of the boat will no longer be above
the water and the boat will sink.

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
1,000,000kilograms 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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