Flying ants are the male and female reproductives (alates) from the fire ant colony. Alates are ants with wings. After mating, the female reproductive becomes a new queen and will establish a new fire ant colony.
These winged fire ants are attracted to reflective surfaces. The shiny water in your pool makes a great landing target.
Find more information about fire ants in eXtension's Imported Fire Ant Resource Area.