This summer has set many records for temperature and lack of rain. The only water sources for wasps are manmade such as water features in public and private gardens. These are magnets for wasps.
Open water attract paper wasps which can congregate in large numbers to drink and collect water. They use the water to manufacture the paper-like material that is used to build their nest. Paper wasps can land on the water and rest on the surface without drowning thanks to surface tension. This force is stronger than the gravity which is pulling the wasp down. This works because the weight of the wasp is spread over six legs each of which has a relatively large surface area. The effect of this can sometimes be seen in the wasp’s shadow.
http://www.delta-optimist.com/opinion/blogs/deltabc-1.983313/blog-wasps-walk-on-water-1.2024544