The term water bug can be applied in different ways. In one sense, a water bug is classified as another term for an American or Oriental cockroach. This type are reddish-brown in colour, and are about 1.5 inches long in size, and although they have wings, they very rarely use them for flight. Their diet is predominantly based around decaying organic matter, but as scavengers will eat any available food. In North America they are most commonly found in steam heat tunnels within large buildings, as well as preferring to build up in open spaces, as well as basements and moist areas. They might also be found in sewers and drainage tunnels, as well as in any outside area with a source of moisture and plenty of shade. The term water bugs can also refer to a subset of aquatic insects within the Heteroptera order, as well as the infra-order Gerromorpha and Nepomorpha. These also habituate damp conditions.
Water bug lives in the water while cockroach crawls on land, that is the only difference: Their place of habitat, where they live.