Bearded dragons can drink tap water; however, it is a good idea to use one of the commercially available dechlorinating products to treat the water.
There are chloramines and chlorine in the water that you want to remove as well as unwanted minerals present in the water. Using these also provides essential ions and electrolytes that are necessary for your pet's hydration. Water conditioners such as Reptisafe can help with this problem, and can be purchased here.
You can also use filtered bottled water in which the harmful chemicals have been filtered out to provide a safe option for hydrating your pet.
Dragons are native to a dry climate and are well adapted to dry conditions, but conditions in captivity are different, and a clean source of water is necessary. A majority of dragons will not drink from a still-standing water source such as a water bowl inside their enclosure, having a water bowl in their cabinet can also increase humidity which can make them sick with a respiratory infection. Because water bowls increase humidity, if you choose to use one, you must monitor the humidity level in its habitat to ensure it stays within the range of 30% - 40%. But this is why a lot of owners choose to use an alternative way to offer water to their dragon such as using a dropper or spray bottle daily to ensure their dragon is hydrated safely.