Fusion between human and water bug. This heavy-duty hybrid's aquatic adaptations are equally practical in a wide range of hostile conditions; storing days worth of oxygen in specialized rectal sacs, the creature can hermetically seal its unbelievably durable shell and function comfortably from the vacuum of space to the abyssal trenches of the sea, resisting extremes of pressure, temperature, energy bombardment and chemical agents alike.
The Exonaut is strictly carnivorous, injecting a paralyzing digestive enzyme through its beak and drinking the liquefied contents of prey. While happiest in a fluid environment, its hybrid anatomy does not lend itself well to active swimming.
I've had this idea for years, but it went through so many revisions that I just didn't like one bit. Very happy with it now. For a long time I wasn't sure what a waterbug hybrid could even really do, but I figure its ability to stay underwater makes it tough against all sorts of attacks.