Hi Pierre, here is the info on Smith/Sandwell from the m_map homepage (see below).

A recent new bathymetry with approximately 1km resolution in lower latitude areas is being used by many people. This dataset is described at http://topex.ucsd.edu/marine_topo/text/topo.html and is available as a 134Mb binary file at ftp://topex.ucsd.edu/pub/global_topo_2min/topo_8.2.img The authors have included an m-file (mygrid_sand.m) which can extract portions of the data. Once this database (and the m-file) is installed on your computer, you can use it in M_Map very easily.