Simplified sea level for a stationary uniform Earth covered by water. Source
Gravity does change slightly in different areas based on Earth's density (more info in this answer). A good explanation of why water level is "flat" is found here.
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