I'm only a year in to backyard beekeeping but from what I understand bees need the water to cool their hive on hot days, so there is likely a hive near by. Once they find a good source of water they will keep coming back to the same spot with their sisters. They take the water back to the hive and start fanning the air to cool off with evaporation.
Hopefully more knowledgeable members have some easy methods for discouraging them. I would suggest making sure they don't have an easy landing place to stay while sipping water (pool noodles or toys floating, etc). They don't get in the water and drown easily, so if there isn't an easy/safe place to land next to the water they'll move on. Maybe if there was more water surface movement it would be too risky to be worth stopping by.
For my own bees I keep a water source between their hives and my pool and haven't had any issues yet. FWIW honeybees aren't very defensive when left alone except for right by their hive...I know this doesn't help if they are always flying around you and your guests.