As I understand it, materials that are known to be extremely toxic are already transported in sealed containers to protect the carriers. However, containers may also contain certain drugs or pesticides that don't fall into this category. If containers are damaged and there is a spill of such products, there could be an environmental impact.
Consider not only toxic products that are already recognized as toxic, but also consumer goods or recycled goods that are transported from one location to another. If they end up in the environment, there will be an impact. If they aren't as toxic as liquids, which are immediately diluted in the ocean mass, they will still have an effect on the environment in the short, medium or long term.