I have lived in the natural range of central bearded dragons when I was working as for a mining company at their direct reduced iron plant, and the area can go for a decade or more without there being any standing or running surface water apart from at bores and artesian springs. These are very rare and usually very remote.
Wild bearded dragons can go their entire lives with out ever encountering running or standing water or heavy rainfall, ie over a decade. The only moisture they will encounter is from fogs and mists and the occasional show (when a degraded tropical cyclone passes) or from the herbs, fruit and flowers they eat or from the insects and other small animals they catch and eat.
Bathing a bearded dragon is not necessary unless :
-- it's soiled itself
-- it's too hot (and needs to cool)
-- it has an injury and requires a soak in a very weak butadiene solution
-- it's impacted (sometimes a luke warm soak will help get things moving)
-- it's in shed and has some stubburn shed on it's toes, feet or tail,
bathing can soften and help this come off.
So
bathing should be a very rare activity for a bearded dragon, none of my beardies have had a
bath this year at all.
Bathing can be very stressful for a bearded dragon too, another reason to not force baths on your pet.