There are two main reasons behind water pollution they are natural or originating in human activity. Any water can become polluted no-mater how big or small nor how deep or shallow it is. It is worth knowing that the bigger the water body is the faster it will regenerate. Water has natural ability to recycle and clean the pollution away.
Sewage and Wastewater
We are billions of people on this planet and we all generate sewage water waste. Dealing with sewage water is no problem for the developed world. As there are plenty of pipe lines that take the water from your home to the water treatment plants. This however isn't the case for the developing world. Which can create problems with health and hygiene.
The industrial accounts for a massive account of pollution on our blue planet. Mostly because a lot of factories aren't up to the standard they should be. Huge amount of waste are generated by these factories as they use fresh water to clean their facilities and drain the water away. This polluted water more often then not returns back to the ground polluting the area.
Which means polluting our marine life, the seas and the Oceans. Millions of people enjoy their time at the beaches around the world. Quite often these a beach goer will leave wast behind. There are places in the world like Lebanon Garbage crises, where human wast is duped right into the sea. This creates massive pollution issue for all of the world.
Radioactive or Nuclear Waste
Well we all heard of the Chernobyl disaster or the more recent one the Fukushima one. Sadly dough these are not the only bad doers. Operating a Nuclear plant generates massive amounts of radioactive fast, so dose mining uranium and other toxic materials. This kind of pollution dose not go away easily and is near impossible to contain once made.
Underground Storage Tanks
Lot's of bad liquid is stored underground in tanks. Many of these tanks has been made several decades ago. On top of that not all of these are looked after correctly. To add to our sorrow they have been made back in a day with materials that often didn't suite the propose. Like the steel tanks, left exposed to the nature, rusting away. Then there are people error while operating these tanks. It all sums up to one more polluter to our planet.
When you have just enough food it is great, when you have too much food you can kick the bucket. This is what is happening with the plants in water. In recent history we have increase our dependence on nutrients and nitrates that we use in agriculture. This adds to us humans polluting ever more and more waters with excess waste. As you can expect this excess waste gets back to the ground water, the rivers and the seas.
All of those gases that we add to the air, with our cars, factories and industries. This all gets picked up by the rain and sent back to the ground, which eventually ends back into water bodies. Read how this happens in our Water Cycle article.
Pollution by drilling Oil
This can be caused in several ways. The ships them selves dose every now end then crack open or run a shore. As much as these make the news and sounds unbelievable in the 21 century this isn't the biggest issue. There are more spills that reach the ocean from every day to day work done by the oil rigs and the tankers. Hopefully this will be resolved as humans move more into renewable energy.