Fresh water is crucial for pets
Now that warm weather is here, I cannot stress how important it is to provide fresh water served in clean receptacles at all times. This is the most important thing for pets, apart from keeping them out of the sun to prevent heat stroke.
One of the biggest differences between pets and wild animals is that the process of domestication has reduced pets' ability to drink dirty water without getting sick. I always find it amazing when I watch nature shows on TV and see a wild animal in Africa walk up to a mud hole full of who knows what and take a long drink from it -- and walk away unscathed.
If any pets drank from that, it would result in an emergency trip to the vet. Any bowl of water left sitting out in hot weather can grow bacteria that can be just as deadly as the water in that mud hole.
My rule is: If you wouldn't drink water from that particular bowl, then don't expect your pets to drink it.