There is a single species of British truly aquatic spider, which live most of their lives under water, Argyroneta aquatica, the Diving Bell Spider or Aquatic Spider. The two species of Raft spiders, Dolomedes sp., one an endangered species in the UK, are semi-aquatic, and live in marshy places and hunt on water, diving if they feel in danger. Wolf Pirate spiders, Pirata sp. as I mentioned earlier, also associated to water and their protective silk tube built on sphagnum mosses ends by the water surface. All these species are unusual in which they have hydrophobic body surfaces, with short hairs that repel water and in the case of the aquatic spider, form a thin coating of air when the spider dives. Gail Stratton and her colleagues studied the locomotion behaviour or 249 American spiders of a 42 families to understand their evolutionary basis. The researchers placed the different species on water surface and recorded their movements. Some spiders sunk immediately. Other spiders which were able to float sometimes used a specialised gait, called 'rowing' which consists on keeping the first and fourth pairs of legs immobile, and moving the middle two pairs simultaneously as propellers, in a similar way to a pond skater. Some spiders able to float would walk as they would do on land. Stratton and colleagues plotted their result on a phylogenetic tree to understand if hydrophobicity and rowing behaviour have evolved just once or more times. You would naturally think that aquatic spiders have evolved their hydrophobic nature in order to be able to live in, on or near water, but in reality, they belong to a small group of spider families in which most if not all species are naturally hydrophobic, and can run - or row - on water. Two of these families, which are closely related, Pisauridae - the family to which marsh spiders belong - and Lycosidae, the wolf spiders, include many species that, although not normally associated to wet habitats, have no trouble walking on water if they need to. Hydrophobicity seems like a prerequisite for regular association to water and later semiaquatic habits.
I wanted to check this by myself with the wolf spider living in my garden, which is likely to be Pardosa amentata, and captured an individual to test it. I placed it on a lily pad in our bath pond and it had no hesitation on walking on water and no trouble running away. In fact, it was so fast that I was unable to take a video of it doing it!
Pardosa sp. hunting on the pond of the wildlife garden
This one is my garden spider demonstrating how its legs rest comfortably on the water surface.
STRATTON, G., SUTER, R., & MILLER, P. (2004). Evolution of water surface locomotion by spiders: a comparative approach Biological Journal of the Linnean Society, 81 (1), 63-78 DOI: 10.1111/j.1095-8312.2004.00269.x