The Online Etymology Dictionary reads:
flounder (v.) 1590s, perhaps an alteration of founder (q.v.), influenced by Dutch flodderen "to flop about," or native verbs in fl- expressing clumsy motion.
Thus, the origin of flounder is not certain. However, after reading that dictionary entry, I wonder if flounder can be simply an embellishment of founder, where the sound unit fl might be used in order to carry a subliminal meaning of heavy movement, according to J. R. Firth's theory.
Can this origin be realistically considered?