Harmful Algal Bloom and Hypoxia Research and Control Amendments Act of 2017 This bill amends the Harmful Algal Bloom and Hypoxia Research and Control Act of 1998 to reauthorize for FY2019-FY2023 the national harmful algal bloom and hypoxia program and the action strategy of the Inter-Agency Task Force on Harmful Algal Blooms and Hypoxia. (Hypoxia is a deficiency of oxygen.) The task force must include a representative from the U.S. Army Corps of Engineers. Each required scientific assessment of harmful algal blooms in coastal waters must examine freshwater harmful algal blooms that originate in freshwater lakes or rivers and migrate to coastal waters. In administering the program, the National Oceanic and Atmospheric Administration (NOAA) must provide: (1) grants for accelerating the utilization of effective methods of intervention and mitigation to reduce the frequency, severity, and impacts of harmful algal bloom and hypoxia events; and (2) technical assistance to regional state, tribal, and local governments with respect to harmful algal blooms and hypoxia events. NOAA must use cost effective methods in carrying out the Act and develop contingency plans for the long-term monitoring of hypoxia. Federal officials may determine whether a harmful algal bloom or hypoxia event is an event of national significance and give funding to the affected state or local government for assessing and mitigating the detrimental environmental, economic, subsistence use, and public health effects of an event of national significance.