As seas have no barriers, pollutants like oil can spread very quickly to different waters around the world.
What are the long-term effects?
Pollution is having an impact on the biology of sea creatures. Scientists researching sea life along the UK coast have discovered female eggs inside male bodies and they think this is the effect of chemicals in the water.
What's being done?
The Marine Conservation Society and various other organisations are trying to make a difference. Last year over 2,000 volunteers collected over 250,000 pieces of rubbish from UK beaches.
A group of fishermen in Shetland have set a scheme to take any rubbish that gets caught in their catch back to shore, instead of just putting it back in the sea.