Which of the following substances is not allowed in US waters?

Oil dispersants are specifically regulated in US waters due to environmental concerns. These substances are designed to break down oil spills, but their use can have significant negative impacts on marine ecosystems and water quality. Regulatory agencies such as the Environmental Protection Agency (EPA) have stringent guidelines regarding their usage to prevent harm to aquatic life and habitats.

In contrast, fuel additives, paint thinners, and wastewater are substances that can be managed under certain regulations. While they can pose risks to the environment, there are provisions in place for their use, treatment, and disposal that allow them to be present in US waters under controlled circumstances. Therefore, oil dispersants stand out as a substance that is not permitted for general use in US waters due to their potential for longer-term environmental damage if used improperly.