I guess that is the first thing you need to check! But it depends on what information you are using (not the GEBCO or ETOPO, but the one you are trying to match it to). Please note that global bathymetry datasets are good as long they are not defining your main area of interest!

Thanks a lot for your consideration and supportive hints

Test with this data:

https://coastwatch.pfeg.noaa.gov/erddap/griddap/srtm15plus.graph