A flounder is a flat fish with both eyes on one side of its head; and, as a verb, to flounder is to wobble around like a fish out of water.Flounder is healthy. ... Above all, beware of taking high doses of flounder, just like any other fish like catfish, salmon, crab, and tilapia; flounder contain some mercury, which may harm pregnant women and children at their tender age if they are taken in high dosages.