Like a horse, or a fish
FOUNDER: verb - to sink below the surface of the water; to cave in; to stumble, becoming lame; to fail. Used especially of horses, livestock and ships. From the Middle English foundren, meaning to sink to the ground.
FLOUNDER: verb - to fling the limbs and body in a failed effort to move; to struggle (as a horse in a mire or a fish on land) to roll, toss and tumble.