Definitions of water flea:

• noun:   minute freshwater crustacean having a round body enclosed in a transparent shell; moves about like a flea by means of hairy branched antennae
• noun:   minute free-swimming freshwater copepod having a large median eye and pear-shaped body and long antennae used in swimming; important in some food chains and as intermediate hosts of parasitic worms that affect man e.g. Guinea worms