I float on the water, back to front, I am the same, come to think of it, so is my name. What am I?

Question

On six feet, I am a noxious drink,
Of whose effects you shudder to think.
Change only my second foot, and then
You convert me into a horrible den
Where the culprit, who gave the noxious drink,
Awaits the fate of which you shudder to think.
What am i ?