Upon the petition of any person not served by an existing transportation line or by a transportation line for which a certificate has been issued, the Commission may cause the same to be set for hearing and give due and proper notice in writing at least ten (10) days prior to the date of hearing to all persons holding certificates nearest to those requesting service and to the municipal officers of all municipalities served or to be served by such water transportation companies, and by publication once a week for two (2) consecutive weeks in a newspaper or newspapers of general circulation in the county or counties in which the line is proposed to be constructed. If at such hearing the Commission finds from competent evidence that the public convenience and necessity require the furnishing of water transportation service to the petitioners, it shall have the further power to order and direct the person holding a certificate who can most economically extend its service so as to furnish efficient water transportation service to the petitioners to so extend its line. It shall not make an order so requiring unless it finds, from substantial evidence, that the person so ordered to serve is earning a fair return on the fair value of its water transportation line and the rendition of service to the petitioners will not prevent the person so ordered to serve from earning a fair return on the fair value of its water transportation line.