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.

A water mite.

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.