The surface of water in a lake is NOT always perfectly level in the sense of being geometrically flat. It is just “level” in the sense of not having any transverse gravitational field, and follows the curvature of a gravitational equipotential which at the Earth’s surface bends downwards about eight inches per mile.
This is obvious if you ever look across a large lake towards a distant city with tall buildings, but is also possible to notice when swimming in a much smaller lake with your eyes just an inch or two above the surface.