Fish-eating spiders are found on every continent except Antarctica. They use surface tension to run on the surface of the water, hunting insects, small fish, tadpoles, and anything else that lives at or near the surface.
The spiders can walk on the water by using their paired legs in a manner not unlike boat paddles, or they can even dive. When they go underwater, a bubble of air becomes trapped by the hairs around their abdomen and becomes their “diving bubble” until they resurface. They have been observed diving to catch underwater prey. (They can also use the wind to propel themselves across the water, using their legs or their bodies as “sails”!)
The six-spotted fishing spider (Dolomedes triton) is named for the clear lines of spots on its abdomen. It is a member of the family Pisauridae, commonly called nursery web spiders — not because they build webs in human nurseries, but because they build “nursery webs” above the water line in which to deposit their egg cases. The young spiders grow up in the web and then move off on their own.
Juvenile six-spotted fishing spider (Dolomedes triton)
The spiders in these photos are only about 10mm across — not quite big enough to catch a fish yet — but they’ll grow to be a couple of inches across, including their legs, when adult. Females are larger than males.
National Geographic had a neat article on fishing spiders in August 2014.