Bathymetry is the measurement of the ocean depths. Although is has no direct use for altimetry data processing, it can be useful for shallow water (since ocean altimetry is not at its best there), or to compare with some ocean features, like currents.
Bathymetry can be computed using among other things altimetry sea surface heights (via the geoid) measurement (see Use Cases: Geophysics: Bathymetry).

fig 1. ETOPO1 Amante, C. and B.W. Eakins, 2009. ETOPO1 1 Arc-Minute Global Relief Model: Procedures, Data Sources and Analysis. NOAA Technical Memorandum NESDIS NGDC-24. National Geophysical Data Center, NOAA. doi:10.7289/V5C8276M

 

The (I)GDRs provide a parameter bathymetry that gives the ocean depth or land elevation of the data point. Ocean depths have negative values, and land elevations have positive values. This parameter is given to allow users to make their own “cut” for ocean depth. It is not a direct computation from the data within the same dataset.