echo sounder

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A device for measuring depth of water by sending pressure waves down from the surface and recording the time until the echo returns from the bottom.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A sonar device used for echo sounding

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a measuring instrument that sends out an acoustic pulse in water and measures distances in terms of the time for the echo of the pulse to return