maison.mousse wrote in sci.geo.geology:

Since the sea surface conforms to the
earth's gravitational field, MSL also has slight hills and valleys
that are similar to the land surface but much smoother.

Yes, the mean sea surface (a.k.a. mean ocean topography) conforms to the
gravitational potential surface to within about 1 meter. Ocean topography
is also affected by long term conditions such as prevailing winds,
currents, temperature, and salinity -- factors in addition to the
gravitational field. The Geoid is defined by the gravitational field
alone.

There are efforts to quantify the differences more accurately.

--
Jim Irwin
http://www.holoscenes.com