Section 39A. A town, by a majority of its voters present and voting thereon at a town meeting at which the voting list shall be used, may establish a water supply or water distributing system and maintain and operate the same, in accordance with sections thirty-nine B to thirty-nine G, inclusive; but no such system shall be established to supply water in any town while the inhabitants of any part thereof are being served directly by a water company or a water supply district, water district, or fire district supplying water to its inhabitants, except in accordance with section thirty-eight or with special law. Any town may vote to authorize its board of selectmen to act as water commissioners, with all the powers and duties of such commissioners, until water commissioners shall be elected as hereinafter provided. A town which has so voted may, at an annual town meeting, or at a special town meeting called for the purpose and held at least thirty days before the next annual town meeting, vote that at such next annual town meeting water commissioners shall be elected.