Even if you took away the waves, the ocean surface would not be flat.  That’s partly because Earth’s gravity varies across the surface, pulling water around and creating huge lumps and holes up to 100m high/deep. But then temperature, salinity, atmospheric pressure and currents add extra dynamic bumps on top of this of a few tens of cm.  Tracking these lets us measure ocean currents from satellites, which is an astonishing feat.
 
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