The US Defense mapping agency topographic center published a paper International Hydrographic Review, Monaco, LIII (2) entitled On the Mystery of Mean Sea Level Slopes.
Researchers had found anomalies in the (averaged) sea levels along the US coastline.
“That in itself would be nothing to be excited about, since it is agreed that sea level is not necessarily level, although nearly so. Even the magnitude of the slope (less than a metre over the length of the coast) is acceptable, except for one disturbing fact: the oceanographers say that this is all wrong, because their data tell them that the slope must go the other way. Specifically, oceanographers and geodesists disagree on the north-south slopes, but agree in the east-west directions. Specialists on both sides of the fence are looking for errors or improvements in the observation — or reduction — procedures, but so far they have not found anything significant enough to resolve the dispute.”
The authors also note that worldwide, there are variations approaching 3.5 meters. As yet unexplained
Note. This paper was published in 1976, and it's likely that the anomalies may have been resolved. See in particular a 2013 study study of the UK coastline which claimed that previously reported 'slopes' were in fact due to errors in measurements.