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]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "bathymetry":
   altimetry, aquiculture, bathometry, biometrics, biometry,
   cadastration, cartography, chorography, craniometry,
   depth sounding, echo sounding, echolocation, fathomage, fathoming,
   geodesy, geodetics, goniometry, hydrography, hypsography,
   hypsometry, marine biology, mensuration, metrology, oceanography,
   planimetry, psychometrics, psychometry, sonar, sounding, soundings,
   stereometry, surveying, thalassography, topography, water