These large and impressive hemipterans are aquatic and are also voracious predators. They exhibit a variety of design features that adapt them for their particular ecological niche. The eyes are large and bulbous, the better to see prey with. The back pairs of legs are flattened and function as oars to propel the bug swiftly through the water. The front pair of legs is large and powerful, with claws at the tip for grasping prey. The most common prey of the GWB is other aquatic insects and invertebrates, but they are also able to feed on small fish and tadpoles. The mouthparts are modified to form a "beak" (this is true of all hemipterans). The beak is especially well adapted for piercing prey in the predatory hemipterans, of which there are many species in addition to the GWB.

Giant water bugs inhabit a wide range of aquatic habitats, including small fast-flowing streams, larger streams with quiet pools, and standing water such as ponds, marshes, and lakes. GWBs spend much of their time submerged, and can stay down for long periods by carrying bubbles of air under their wings. Their breathing tubes open where the bubbles are, and gasses can be exchanged with the air in the bubbles. Insects accomplish gas exchange in a way that is very different from vertebrates. In vertebrates, gasses are exchanged with the environment through lungs or gills, and the blood carries the oxygen to the tissues. In insects, the body is permeated with small air-filled tubes (the tracheal system), so that molecular oxygen and carbon dioxide are exchanged directly at the tissues that use them, thus eliminating the need for a blood vascular system. The tracheal system works very well for small animals like insects. In the GWB, oxygen in the bubble can diffuse into the tracheal system, and CO2 in the tracheal system can diffuse into the bubble. From the bubble CO2 can diffuse into the water, which maintains the concentration gradient for more CO2 to diffuse. Oxygen goes in the other direction. As oxygen is removed from the tracheal system by the tissues (which are metabolically active), O2 from the bubble diffuses into the tracheal system. As the O2 in the bubble is depleted, more diffuses into the bubble from the water. This system (carrying a bubble of air when submerged) is referred to as a "gas gill", and it is found in many aquatic insects, both in species that are phylogenetically closely related to the GWB and in species that are phylogenetically distantly related to the GWB.

Giant water bugs have a somewhat unusual mating system. The male provides parental care for developing eggs by allowing a female to glue eggs to his back. In this way he can protect them from predators, aerate them periodically, keep them moist if necessary, etc. But having eggs glued to his back prevents the male from flying, so places him at some risk (of predation, or inability to escape from harsh environmental conditions). So the male wants to be sure the eggs glued to his back are fertilized with his sperm. Female GWBs mate with more than one male because their reproductive success is limited by access to male "back space". Like most other female insects, female GWBs store sperm in their reproductive tract. Thus males are faced with the possibility of cuckoldry, and they engage in a simple form of sperm competition called "sperm displacement". Males copulate repeatedly with a female while she is in the process of laying eggs on his back; copulation occurs after about every third egg. Each copulation pushes any previously deposited sperm farther away from the eggs to be fertilized, which is why this repetitious copulation strategy is called "sperm displacement".