water bug

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

• noun Any of various aquatic insects, especially a water boatman, a backswimmer, or a large predatory hemipteran of the family Belostomatidae.
• noun A large cockroach.

from the GNU version of the Collaborative International Dictionary of English.

• The Croton bug.
• Any one of numerous species of large, rapacious, aquatic, hemipterous insects belonging to Belostoma, Benacus, Zaitha, and other genera of the family Belostomatidæ. Their hind legs are long and fringed, and act like oars. Some of these insects are of great size, being among the largest existing Hemiptera. Many of them come out of the water and fly about at night.

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

• noun Any member of the infraorder Nepomorpha, the true water bugs.
• noun Any of various other true bugs that live around water.
• noun The American cockroach, Periplaneta americana.
• noun The Oriental cockroach, Blatta orientalis.

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

• noun a true bug: large aquatic bug adapted to living in or on the surface of water
• noun small light-brown cockroach brought to United States from Europe; a common household pest