Any of various long-legged insects found floating or moving swiftly on the surface film of water, such as a crane fly or pond skater. Now colloquial.

any of numerous mites adapted to living in water, typically acting as predators or parasites of other aquatic animals, esp. invertebrates.

†(a) a biting or bloodsucking parasite found in marshy areas (probably the larva of a trombiculid mite) (obsolete rare); (b) = water mite n.