Mass. Gen. Laws ch. 111, § 173A

Police employed by the water board or board of water commissioners of a city, town or water district, a public institution or water company, and duly appointed in one of the cities or towns within an area from which water is being supplied under the direction of such board, institution or company, or over which such board, institution or company has control, or where such water is furnished by such city, town, district, institution or company shall within and throughout any or all of such areas have all the powers and duties of police officers of cities and towns.