Laws need to be responsive and adaptive to the often-unforeseen changes in the marine environment, taking into consideration the marine ecosystem as a whole, and moving away from the traditional sectoral management. This may weaken the rule of law. Central to the legislative instruments in force regulating the use of aquatic systems and ocean management, is the concept of the ecosystem approach, which is extensively employed by instruments safeguarding marine ecosystems. This Chapter will consider implementation of the ecosystem approach in the overlapping legislative measures regulating ship source oil pollution in the Baltic Sea, a small intracontinental regional sea, which is one of the most polluted seas in the world. It will also consider how institutional interaction between these measures has affected implementation of the ecosystem approach in this regional multilayered regulatory system, and if that interaction will enable regional institutions as the key to successfully regulating international sectors such as shipping and strengthening the rule of law.