Bees will go to a bird bath for a drink of water - but a swarm? that sounds like a queenbee trying to re-locate. Check for a local apiarist, who can probably help you, even to re-locating the swarm to better quarters.

BTW, I've found that if you set up a mudding puddle for butterflies and keep it dependably damp, most bees rather go there to get a drink - less liklihood of drowning.

If it has been hot and somewhat dry in your area, and there are few water sources for foraging bees, they will seem to swarm a bird bath or garden pond. Perhaps there is a domestic or native hive that live in the vicinity. A friend of mine has a lovely (small) water feature in her yard that looks like a sea of honey bees in the summer. She and her husband also have several domestic hives that they maintain.

I don't have a real helpful suggestion, other than to provide alternative water sources. I would tend to favor the bees' efforts, I think, since they are in environmental peril in recent years.

I've placed a very inexpensive plastic plant saucer
( suitable for a very large container) in a shady area under some trees, and filled it with a rock and plenty of fresh water daily. It kept a large number of birds happy.