Answer:
What goes through a door but never goes in. And never comes out?
A morbidly-named body of water
In we go, out we go. All around and in a row. Always, always steady flow. When we'll stop, you'll never known. In we go, out we go.
It flies when it's on and floats coming off.
People are hired to get rid of me. I'm often hiding under your bed. In time I'll always return you see. Bite me and you're surely dead.
If it’s information you seek, come and see me. If it’s pairs of letters you need, I have consecutively three. What am I?