What an amazing little animal this is? The body of the female raft spider (Dolomedes fimbrata) can be up to 20mm, over half an inch, which means that with those eight legs nearly an inch long the whole beast is over 2 inches long from front to back tips.
The raft spider, as its name implies, is only found near water, inhabiting swampy areas that have larger clear pools amongst them. Although widespread they are much commoner in the south of England and are, because of their particular habitat requirements, quite local. The marshy areas around Arne RSPB in Dorset is as good a place to see them as anywhere. Look for lilly pads on the many 'dragonfly' ponds and then look for the spider waiting on it with its front four legs touching the water's surface. When they detect an insect struggling in the water they run across the water and strike.
The females have a rather nasty habit of eating their partners after mating so, from a personal point of view, I am quite glad I was not born a raft spider.
Seen in Spring and summer, to my mind they are worth a visit to Arne alone!