“Why, this ground moves!” cried Charley, astonished.

“You'd better look out!” cried Joe. “It won't hold you up much longer. It's not ground at all; it's floating moss and stuff —”

The speaker paused suddenly, as Charley now broke through, and stood in mud and water nearly up to his waist.

What is “blackwater” anyway? This blog post explains the famous midnight-black water of the Okefenokee Swamp and the Satilla River.