What agreement oversights pollution over the entire ocean?

Wikipedia says:

"The United Nations Convention on the Law of the Sea (UNCLOS), also called the Law of the Sea Convention or the Law of the Sea treaty, is the international agreement that resulted from the third United Nations Conference on the Law of the Sea (UNCLOS III), which took place between 1973 and 1982. The Law of the Sea Convention defines the rights and responsibilities of nations with respect to their use of the world's oceans, establishing guidelines for businesses, the environment, and the management of marine natural resources. The Convention, concluded in 1982, replaced four 1958 treaties. UNCLOS came into force in 1994, a year after Guyana became the 60th nation to sign the treaty.^[1] As of January 2015, 166 countries and the European Union have joined in the Convention."

That's a lot of territory! We'd like to focus on the marine environment for a moment. The International Maritime Organisation (IMO), in consideration of a major industry on the sea:

"Shipping – which transports about 90% of global trade – is, statistically, the least environmentally damaging mode of transport. Moreover, set against land-based industry, shipping is, overall, a comparatively minor contributor to marine pollution from human activities." 

see more here

They further advise that there are 51 treaty agreements for shipping here and 21 relate to the environment.

The Marine Environment Protection Committee (MEPC) is IMO's senior technical body on marine pollution related matters for instance the International Convention for the Prevention of Pollution from Ships (MARPOL) in 1973.  

MARPOL has been instrumental in reducing the number of oil spills and a wider range of measures to prevent marine pollution including pollution from chemicals, other harmful substances, garbage, sewage and air pollution and emissions from ships.

Compliance on an international scale relies on all countries working together to ensure standards are met. There is some debate about compliance here and also thoughts about how to mitigate increased shipping emissions for the future here .