Final answer:
Government regulation of agriculture is the most effective method to decrease marine pollution.
Step-by-step explanation:
To decrease marine pollution, the most effective method would be government regulation of agriculture. Agricultural runoff is a major source of marine pollution, including excessive nutrients and pesticides entering water bodies. By implementing regulations and practices to control agricultural pollution, such as managing fertilizer use and preventing soil erosion, the amount of pollutants entering the oceans can be reduced. This would help protect marine ecosystems and organisms from the harmful effects of pollution.