Around the farm I have numerous mini ponds used primarily for mosquito control. They're all stocked with fish known to eat mosquito larvae. It didn't take me long to notice that my bees were using the ponds as a water source and sadly, many were drowning. But once pond vegetation was added, the bees had enough footing to prevent them from falling into the water.
About a year ago I introduced azolla into ponds, mainly because the maintenance was far easier. Much simpler to harvest excess pond vegetation with a dip net, as opposed to hand hauling out mats of tangled plants and hacking them into pieces for removal.
The azolla covers the pond surface with low growth that the bees seem to prefer. They easily walk over its surface and have simple, safe access to the water below.