Dictionary.com
definitions

hydrospace

[hahy-druh-speys]

noun

the regions beneath the surface of the oceans and seas.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
hydrospace in Science

hydrospace

[hī′drə-spās′]

The regions beneath the ocean's surface, especially when considered as an area to be studied, as in marine mammal research or sonar mapping of the ocean floor.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.