"When you have $1 million, you don't see a loss or gain of a cent"..
Water has a density of 1 g/cm^3, while solid rock has two to three times that (or more if it contains more metals). But this is only a small "skin" on top of the Earth. The deepest ocean has a trench 11 km deep, while the Earth has a radius of 6730 km. 11/6370 = 0.17% (in mass it will be significantly worse, calculate how much using the average density of the Earth)
Apparently, you haven't bothered to find out how little difference we are talking about..
Do you have any data to support that something you perceive as flat is flat? To you, it appears flat because you are big and your eyes are imprecise. Under a microscope, a flat surface is full of "mountains" and "valleys." These "mountains" are "big" for microns or nanometers.. (i.e., irrelevant to the human body counted in meters)