This a Raft Spider, one of two species found in the UK. The other is very rare and classed as endangered, and only found in a handful of sites.
Raft Spiders spend most of their time living and hunting on the surface of the water.
They can swim though and will dive underwater to catch small fish.
If you look carefully between her ( females are the larger and this was quite large), two upper most right legs, you might just spy that she appears to have caught her dinner.
