The Collaborative International Dictionary of English v.0.48:

Bathymetry \Ba*thym"e*try\, n. [Gr. ba`qos depth + -metry.]
   The art or science of sounding, or measuring depths in the
   sea.
   [1913 Webster] bathyscaph

WordNet (r) 3.0 (2006):

bathymetry
    n 1: measuring the depths of the oceans [syn: bathymetry,
         plumbing]