Water System Threat Preparedness and Resilience Act of 2023 This bill requires the Environmental Protection Agency (EPA) to create a program to support increased membership and involvement of certain smaller water utilities and water treatment works (e.g., wastewater systems) in the Water Information Sharing and Analysis Center (WaterISAC). As background, WaterISAC is a group of water and wastewater systems and associations that coordinate with the EPA and other federal agencies to collect and analyze data on water security and threats. WaterISAC also provides analysis and resources to support response, mitigation, and resilience initiatives. Under the bill, the EPA must develop a program to encourage and support WaterISAC membership by community water systems (e.g., public water systems that regularly serve at least 25 year-round residents) and water treatment works, offset membership dues for community water systems and water treatment works, cooperate and coordinate with WaterISAC to expand incident data collection and analysis of threats related to the water sector, and enhance WaterISAC's tools and resources for monitoring the water sector and increase the preparedness of community water systems and publicly owned water treatment works to identify and respond to malevolent acts or natural hazards.