@thomasvanderlinden : hi - another possible way to go is downloading global high resolution topo/bathymetric data from gebco (15 arc-second):
Data come in (very conveniently) lat/long and you have elevation in meters. The netcdf file you will download download, in its metadata description, contains all the info and details about measurements and references adopted when creating the dataset.
You can also use a specialized color-map to color land and ocean (asymeetric, diverging).
And also, you can download the high definition coastline to overlap to your topo/bathy data. I am sure you might find this link interesting:
Good luck.
Marco