Sea level manipulations of data, there is NOT alarming rise
Danilo Anton
Sea level changes have been commonplace along geological history.  It is difficult to pinpoint those changes in older sedimentary rocks because the level of emerged lands also varied considerably..
Many times we don't know how high was the depositional sea level if the marine sediments are found on high elevations which were uplifted milliones of years ago. Obviously the level can be assumed through analysis of the geological context  and correlations. However in most cases these are only assumptions. Similar arguments can be used in relation to marine sediments buried under hundreds of meters of other sediments

Even when sediments were not uplifted o did not sink locally it is very difficcult to know what was the sea level worldwide.
It is somewhat easier to estimate past sea level in more modern sediments, let's say Quaternary or Pliocene sediments. However, even in those cases, the vertical movements of continental masses may introduce modifying factors.
Having said that, when sea level changes in a scale of a few years are studied, other difficulties arise. First, there are always vertical movements en ALL coastal shores. These movements may be a few millimeters per year or even less. However we don't know them with precision and it is very difficult to measure those millimetric changes, because it is not easy to find a reference.
On the other hand, sea levels may vary in different oceans due to increased evaporation, flow from neighboring rivers or melting from ice caps or glaciers. Therefore it is hard to know exactly (to the millimeters scale) what is the worldwide sea level.
With satellite measurements it has become much easier to get data from all oceans and seas. However, often these data require some elaboration and they may be manipulated to demonstrate whatever theory want to be demonstrated.
Nils-Axel Mörner has been studying the subject for several decades and described these uncertainties and manipulations in an article entitled "There Is No Alarming Sea Level Rise!" (see footnote reference).
Economic, academic and egotistic interests are providing non-trustable data and it has become increasingly difficult to believe some ot the official assumptions and particularly the catastrophic forecasts of coastal flooding in many parts of the world.

https://www.21stcenturysciencetech.com/Articles_2011/Winter-2010/Morner.pdf