Sec. 4. (a) A district established for the purpose of furnishing water supply for domestic, industrial, and public use may elect to furnish water supply under this chapter if:
(1) the district plan; or
(2) a part of or an amendment to the district plan;
so states.
(b) A district that adds the purpose of furnishing water supply for domestic, industrial, and public use may elect in the manner provided by subsection (a) to furnish water supply under this chapter.