What is leading to the spoilage of oceans and what protection measures have been taken to protect them?

It's a combination of things, mostly caused by man.  The world is carrying over 6.5 billion people at the moment, and that population produces a gigantic amount of waste.  In the last 50 years, but the last 20 years especially, we also produce a ridiculous amount of plastic waste in the form of soda and water bottles.  There is a mass of plastic waste in the middle fo the Pacific Ocean they estimate is the size of New Jersey.

Also, far too many countries have, for years, been dumping industrial, medical and even nuclear waste into the ocean, which is now truly showing signs of human negligence.

Lastly, a major function of the oceans is to absorb carbon dioxide from the air, as part of nature's natural cycle.  With the increasing amount of carbon in the atmosphere, the oceans have become saturated, raising the overall Ph of the water, and killing off coral reefs from Australia to Florida.

There are international laws in place to prevent this kind of pollution, as well as treaties and recycling of plastic is much, much better and more widespread than it used to be.  But the laws have been difficult to enforce.  Some countries have a much better environmental policy and record than others where the oceans are concerned, among them Canada, New Zealand, and Denmark.