Pollution is the leading cause of spoilage of oceans. This pollution is caused by humans. Take the recent oil spill in the Gulf of Mexico for example. Millions of gallons of oil is being spewed into the ocean each day. This is catastrophic in numerous ways. The ocean water is highly contaminated, destroying habitats of thousands of marine animals. In addition, coasts are becoming contaminated and destroying habitats of coastal animals as well. Restaurants that sell sea food are even closing because the food they sell is being destroyed.
There are many guidelines that are in place to help preserve the oceans and keep them clean. Unfortunately, these rules and laws are not always followed. For example, waste material is dumped into the ocean causing polluted waters.
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.