Water spider : An aquatic European spider (Argyoneta aquatica) which constructs its web beneath the surface of the water on water plants. It lives in a bell-shaped structure of silk, open beneath like a diving bell, and filled with air which the spider carries down in the form of small bubbles attached one at a time to the spinnerets and hind feet. Called also diving spider.

Water spider : A water mite.

Water spider : Any spider that habitually lives on or about the water, especially the large American species (Dolomedes lanceolatus) which runs rapidly on the surface of water; -- called also raft spider.

Example:


Taxonomy
bell, Argyroneta aquatica, Diving bell spider, species, Aranea, females, free encyclopedia, diving bell, northern Asia, prey, the bells, air supplies, hunting, Marshall Cavendish Corporation, water spider, prey animals, genus name, The spiders.

Defination from the web:
spider that spends its whole life under water. However it breathes air, which it traps in a bubble held by hairs on its abdomen and legs.[1] ... Adaptations to an aquatic life may be responsible for the reversed sexual size dimorphism in the water spider, Argyroneta aquatica" ...