The very simplest answer to this question is
gravity! Gravity is the force between all
objects with mass that causes them to be attracted
to one another. It pulls us to the earth and pulls
the earth to us. The oceans are also pulled to the
ground by gravity.
The earth is spinning very quickly around its
axis, and it seems as though this would fling all
of the surface water into space (I know that if I
tried to spin a wet basketball on my finger I
would get peppered in drops of water!). However,
the earth is so big, which makes its gravity so
strong, that it would have to be spinning
impossibly fast (nearly 20 times faster than it is
spinning now!) in order for the water to be
accelerated fast enough to break free of the
earth's gravity. Thank goodness!
I hope that this answers your question.
Questions like yours are precisely why
scientists do experiments and try to make theories
about why the world works the way it does.
All the best,
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