Are you POSITIVE there are no mites on this snake? Mites bite & cause irritation- snakes often seek relief in water bowls. Look closely, they're tiny (they hide under scales but may also come off in the water bowl). I agree that a damp cage can also cause scale infections.
Another possible reason that a snake seeks the water bowl is that it's too warm. Double check the temps. accurately- what's are they? Snakes need warmth, but also part of the cage with a lower temp.