Three important things about the water supply -
- must get the sun (my pond doesn't, and the bees take very little notice of it), the birdbath gets sun in the afternoon, and from lunchtime onwards, its their pub. I think its not just about the warmth of the water itself, but also about the sun warming the bees while they tank up on still-cool water.
- must be obvious to the bees. If it blends in with the background, they'll stand a better chance of missing it. Visual or odour cues would both help (we all know about the attractiveness of smelly water, whether in hot tub or septic tank.) Pale stone or black bucket (with pale stone) stand out from the background. I've successfully used potted plants to signpost a watering hole.
- must have a good 'beach' - in the sun, as above. Mud or moss or even greenhouse mesh is good. A sharp clear 'cliff' isn't so good. The more beachfront, the more bees can drink at the same time.