If it only happens at this time of year, they're probably swallows gathering water to build their mud nests, and small flying insects from the surface of your pool to feed themselves and their nestlings.  The pooping is just incidental, because they pretty much eat and eliminate constantly during breeding season.  The little sacks of waste that they deliver to you are gifts from the baby birds.  After feeding the chick eliminates a nice little pouch for the parent to pick up and dispose of to keep the nest clean and help keep predators from discovering the nest site... Why in your pool???  I can only surmise that in nature they would deposit them into a lake or ocean or other body of available water as a way of totally destroying the evidence... It would seem that, to them, your pool is the perfect urban alternative. ; )