Re: Why do Wasp drink water from my pool and what do they need it for?

Wasps, like all creatures, imbibe water to maintain their body’s hydration 
and dilute wastes.  They also use lots of water (or wet mud) to make their 
nests, which are either made from mud (mud daubers and potter wasps) or 
carton (paper wasps and their relatives).  You probably have paper wasps 
from the discription (mud daubers usually are more interested in actual 
mud.)

For a reference on paper-making and coating with water see:
Paper Wasps. http://tn.essortment.com/paperwasps_rwwv.htm

One reference to wasps drinking is on the BBC: Wasps. http://www.bbc.co.uk/dna/h2g2/A225839

For a picture of a paper wasp drinking see: Wasps by Valerie.   http://home.att.net/~larvalbugbio/wasps.html

and: Trek Nature.  http://www.treknature.com/gallery/Europe/France/photo19908.htm