Also found in: Wikipedia.
One trained to pilot, navigate, or otherwise participate as a crew member of a deep-sea vessel.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(Military) navy US a person trained to operate deep submergence vessels
[C20: from Greek, from hydro- + -naut, as in aeronaut, astronaut]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014