The Earth is has a globular shape and sea water level is a plane.
We are on the upper surface of the Earth, then how is sea water level a plane?
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Water level is flat with regard to the balance between several forces predominantly gravity (other smaller effects include baroclinic effects). The direction of Earth's gravity is a combination of gravitational pull and inertia (centrifugal "forces"). The gravitational pull is toward the relative center of mass of the earth-moon system depending on the relative position of the Moon with respect to the local position on Earth's surface.
Simplified sea level for a stationary uniform Earth covered by water. Source
The ocean is not like a bathtub. At shorter time scales the water level surface responds as a balance between multiple forces (tides, wind stress, friction, baroclinic torque, ...). The balance is complex and highly local: Source