I have a two-gallon, galvanized bucket that I keep full of water. I cut a round piece of pine and drilled holes in it for the bees to land on. It sits in front of, and between, my two hives.
I've heard it said that once bees find a water source they tend to stick to it unless it dries up, but I don't know if that's true or not. I don't know where my bees get their water, but it isn't from the bucket as I was a week late getting it in. It makes me feel better to keep it filled, and it's also required by city ordinance where I live.