The original concept of ocean, which means a great river encircling the mother earth, goes back to the time of Mesopotamian and Indo-European civilizations. Samudra from Hindu mythology was imagined to be a celestial water body ruled by Varuna. Actually oceans as we know today are interconnected body of salt waters often referred to as the World Ocean or global ocean though arbitrarily divided into five separate oceans. Global ocean circulations greatly affect Earth's climate by transferring warm or cold waters and associated precipitations to coastal regions.
 
Some useful links:
 

• Ocean Explorer - An educational and reference resource from NOAA
• Oceanography Image of the Day - from the Woods Hole Oceanographic Institution
• Science taps into ocean secrets
• Why is the ocean salty?
• Intergovernmental Oceanographic Commission
• Oceana - Protecting the World's Oceans
• CORE - Consortium for Oceanographic Research and Education
• NOPP - National Oceanographic Partnership Program
• NOSB - National Ocean Sciences Bowl
• CoML - Census of Marine Life
• World Ocean Observatory
• Greenpeace Defending our Oceans
• The Last Days of the Ocean, a Mother Jones special report on the state of the ocean
• Ocean Voyager, a five-part journey to save the seas, created by Mother Jones magazine
• The Ocean Conservancy - Advocates for Wild, Healthy Oceans
• NOAA DChart - Plot and download ocean data from your browser or Google Earth
• UN Atlas of the Oceans
• American Fisheries Society
• Cousteau Society
• Marine Environmental Research Institute
• NOAA National Fisheries
• NOAA National Ocean Service
• Ocean Alliance
• Ocean Futures Society
• National Office for Integrated and Sustained Ocean Observations
• One Fish
• Blue Ocean Institute
• Changing Currents: Charting a Course of Action for the Future of Oceans