When I use blocks of wood or other float to give the bees something to land on, I find they get waterlogged and sink. Also bees falling into the water are seeding the potential for disease, and freestanding, still, water is a mosquito breeding facility.
In using floating water plants and putting Goldfish in the tub, I have confronted these three vectors. Goldfish, being opportunistic carnivores, won't actively hunt the bees, but will eat the drowned insects and mosquito eggs. The floating plants allow them the security of getting down to the water by walking toward the center of the rosette the leaves form. I can also control the quality of the water the bees drink.
The two floaters shown here, Water Hyacinth (Eichhornia crassipes) and Water lettuce (Pistia stratiotes), are semi-tropical, so would have to be overwintered in a greenhouse. Florida also bans the transport and trade in these two plants.
This isn't the only watering source for the bees. I see a lot of them crawling over my potted plants after I water them, and the cuttings in the perlite. There's a pond within 250 feet, but I'm leery about the water quality.My Bee Watering System for the science fair