Mine is just a kiddie pool with floating grass/hay for them to be able to land on and a mineral block inside it. What someone else said about the tray with pebbles works great also. As long as they have somewhere to stand and not drown, they're happy.
I used to have a disappearing waterfall...
Bees like mineral water when it is hot and will eagerly search out swimming pools because of the chemicals. I have two hives and neighbors with pools, and I put out a small kiddie swimming pool filled with water them dump in a small mineral block (like for cows/horses to lick) and floating...