Dolomedes aquaticus is a fishing spider that lives among the stones of braided riverbeds, usually less than 5 metres from the river on plants, stones, or pieces of wood at or near the surface of the water. It’s found around braided rivers and the rocky foreshores of lakes in the South Island and in the southern part but not the north of the North Island (possibly because of the lack of braided rivers there). It can swim and run very rapidly on the surface of the water. When teased, it makes for the nearest object, down which it runs to the bottom of the water, where it remains until the danger has gone.
Its colouring allows it to blend in with river stones. Normally nocturnal, it sits and waits for its prey after dark.
Females have a total body length of 13–26 mm, males being smaller at 11–18 mm. The fourth leg is longest, about 36 mm in females and 33 mm in males. The cephalothorax is chocolate brown, with a supra-marginal band of yellow extending from the posterior slope to the anterior angle of the pars cephalica: falces, maxillæ, labium, and sternum chocolate-brown; legs and palpi, brown; abdomen above greenish-brown with two longitudinal rows of brown-margined yellow spots, at the sides greyish, and below dusky-brown with four more or less continuous longitudinal whitish stripes converging towards the anus. At the base of the dorsal surface there is a short median spathulate band of paler hue than the rest of that surface and on each side of this band a short grey fleck. The cephalothorax and abdomen are densely covered with grey, yellow, and brown pubescence.
While incubating, the female goes some distance from the water and lives under a large stone or a piece of wood. Here she remains till the young are hatched. During incubation, she shows considerable aversion to water.