The Clean Shipping Act of 2023 requires the Environmental Protection Agency to establish standards that dramatically reduce greenhouse gas emissions from commercial vessels operating in U.S. waters. Starting in 2027, vessels must use fuel with progressively lower carbon intensity, reaching a 20 percent reduction initially, then 45 percent by 2030, 80 percent by 2035, and ultimately zero carbon emissions by 2040. The law applies to ships of 400 gross tons or larger making voyages between U.S. ports or between U.S. and foreign ports, affecting major shipping companies and maritime operators. Vessels must report their fuel carbon intensity and emissions annually, and the EPA has until January 2026 to finalize the initial standards, with subsequent standards to be completed two years before they take effect. The legislation includes flexibility provisions allowing the EPA to adjust requirements if meeting them proves technologically or economically infeasible, and permits companies with multiple vessels to average their emissions across their fleet to demonstrate compliance.