bathymetry

n. measuring of the depth of bodies of water

Bathymetry is the underwater equivalent to altimetry. The name comes from Greek βαθυς, deep, and μετρον, measure. In other words, bathymetry is the study of underwater depth, of the third dimension of lake or ocean floors. A bathymetric map or chart usually shows floor relief or terrain as contour lines, and may additionally provide surface navigational information.
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Noun
1. measuring the depths of the oceans
(synonym) plumbing
(hypernym) measurement, measuring, measure, mensuration

(n.)
The art or science of sounding, or measuring depths in the sea.
  

Science of measuring water depths (usually in the ocean) to determine bottom topography.