S. 3736 directs the Secretary of the Interior, working through the Bureau of Reclamation, to conduct a feasibility study on constructing the Dakota Mainstem Water Supply Project, which would deliver municipal, rural, and industrial water to communities in South Dakota, Iowa, Nebraska, and Minnesota. The Secretary must partner with the Dakota Mainstem Regional Water System, Inc., a nonprofit serving as the non-federal project entity, and enter into a cost-sharing agreement in which the federal government covers no more than 50 percent of study costs, with total federal spending capped at $10 million. Within 90 days of enactment, the Secretary must identify funding sources for the federal share, and after the study is complete, must produce a public report recommending whether the project is technically and financially feasible, whether it should be authorized for construction, and what the non-federal cost share for construction (at least 25 percent) should be. This report goes to relevant House and Senate committees and must be made publicly available along with supporting study documents. The authority to conduct the study expires five years after the bill becomes law, and the Secretary is also required to consult with federal, state, tribal, regional, and local authorities throughout the process.