Definitions for water bug
Random House Webster's College Dictionary
any of various aquatic bugs, as of the family Belostomatidae.
(loosely) a very large, relatively slow cockroach, as the American cockroach.
Origin of water bug:
a true bug: large aquatic bug adapted to living in or on the surface of water
German cockroach, Croton bug, crotonbug, water bug, Blattella germanica(noun)
small light-brown cockroach brought to United States from Europe; a common household pest
Any member of the infraorder Nepomorpha, the true water bugs.
Any of various other true bugs that live around water.
The American cockroach, Periplaneta americana.
The Oriental cockroach, Blatta orientalis.
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 Belostomatidae. 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