While sea spiders are arthropods, making them related to land spiders, they are different from land spiders in several ways. Most obviously, they live in water. They also don't spin webs (something that would be difficult to do with the strong currents in the ocean!). They also may have more legs than land spiders, who have 4 pairs of legs.
The sea spider in this image looks remarkably like a land spider. This one is located on a rock on the Davidson Seamount off the coast of California, and was found by researchers from NOAA and the Monterey Bay Aquarium Research Institute.