Eustasy is the mean average of ocean levels, abstracted from local conditions, and today is measured by satellite networks and figured out by computerized algorithm. It's constantly changing. Most obvious is the 24 hour 54 minute tidal cycle brought on by the ocean's relationship to the gravitational force of the moon, but also affected by the sun and other heavenly bodies nearby.
The freezing and thawing of water globally shrinks and swells the ocean by several hundred meters over the epochs. But long term eustasy becomes indiscernable when considering hard-to-trace variations caused by the epierogenic--or bobbing--motion of the continental plates. Likewise, the shoreline travels up and down as the lands move, grow, and collapse. The density, and thus surface, of the ocean changes with tidal influences as well as long term changes in oceanic temperature and large-scale atmospheric and seismic pressures enacted from above and below. The accumulation rate of sentiment at the bottom of the sea varies from minute to minute; the deep seaman's bob gets buried rather quickly.
Again, if we want to believe anything, we must have faith in an unchanging, singular center, in infinitesimal point, a true center of the Earth, from which to pull our virtual tape measures, and read the height of the seas. Is there a gravitational balancing point in the heart of this mass? If so, could this spot be anything if not miraculous?