The waterworks board shall make all appointments and hire all officers, agents, servants and employees in the waterworks plant and fix their compensation and determine their qualification, and shall have regard alone to relative capacity of applicants, their moral, physical and health qualifications, and when appropriate, their qualifications for manual labor. All appointments shall be made on the basis of their merits alone, and no appointment shall ever be made on account of political services or affiliations. Such officers, agents, servants and employees shall hold their offices during the pleasure of the waterworks board.