Nobody in our lab is aware of any study that has looked at the possible toxicity of swimming pool water to bees. However, bees certainly seem to like drinking from pools. It could just be that a pool is an easy water source to find, but we also know that bees generally like a tiny amount of salt in their water. Perhaps swimming pools have the right amount of salt to please the bees thanks to some human sweat or some pH balancing chemicals. These pH chemicals are usually sodium and chlorine based compounds like hydrochloric acid or sodium bicarbonate that are in specific formulations to make them safe for home use, leaving "sodium chloride" in the water. It is clear from many anecdotal observations that bees like drinking out of swimming pools, although they do not like to get their feet wet. Pool water is formulated for it to be safe for us to drink, and we do not know of anything about the physiology of the bee gut to suggest that they would have trouble that we would not. In addition, on hot days bees will recruit large numbers of foragers to gather water rather than food, and a swimming pool is a good place to do so. They then spread this water around the hive and fan their wings the over water droplets to produce evaporative cooling. So, the bees may not even be directly consuming much of the water that they collect. If pool water was toxic to the bees, beekeepers would probably have noticed bee kills by now because large numbers from a single hive would have drank the same water. So it is highly likely that it's safe enough not to worry about. However, to be on the safe side beekeepers may want to consider putting out other sources of water for their bees, particularly during the hot, dry months.