How is it determined when you go from Pacific Ocean to Atlantic Ocean around Tierra del Fuego, Argentina? Or the Atlantic Ocean into the Indian Ocean around the Cape of Good Hope, South Africa?

Is there a region where you could be in the South Pacific, South Atlantic, and the newly recognized Southern (Antarctic) Ocean at the same time?

Since all of the oceans are defined in terms of a specific area, one must conclude that there are very specific dividing lines. This being the case, it is logical to conclude that you are only in one body of water at a time.

According to the CIA World Factbook:

"The decision by the International Hydrographic Organization in the spring of 2000 to delimit a fifth world ocean, the Southern Ocean, removed the portion of the Atlantic Ocean south of 60 degrees south."

The specific coordinate given by this site is not of much use in your quest, I'm afraid. Perhaps this site will provide a start (though it may predate the info given above). If you find the exact boundaries, would you post them here? I will do the same.

...'fuse

Coldfuse, your second site is very general. I noticed that the Pacific Ocean boundaries also include the Sea of Japan, the Philippine Sea and the South China Sea.

I suppose the question is difficult to answer not only because water doesn't easily divide into sections, and also because navigation instruments are not absolutely accurate. Even if you could note the boundaries on a map, it would be impossible to know exactly when you've crossed the line. (The two most accurate ways of navigating blue waters--GPS and pitswords--still have margins of error large enough there to be a gray area in transit.)

It seems to me that it would sorta be a common sense type of defination: such as an imaginary line going south from the souther tip of Argentina-to the right the Atlantic, and to the left the Pacific...Holds true at the southern tip of Africa--to the right the Indian Ocean-left the Atlantic...the East boundry of the Indian Ocean would be an imaginary line curving from the Sunda Islands (Sumatra) down the west coast of Australia....
I have never seen a defining line stating the exact location of the Ocean borders...