The OED tells us this one is onomatopoetic from blending such sounds and senses as “flop”, “blunder”, and “flodder”, an obscolete word meaning “flood”.

• 10.012 in the darkness floundering

“† ˈflodder, v.” OED Online. Oxford University Press, June 2015. Web. 26 July 2015.

“flounder, v.” OED Online. Oxford University Press, June 2015. Web. 26 July 2015.