In order to aid scientists in studying and understanding the vast seas of the Earth, researchers generally divide them into four distinct oceans. These oceans are the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, and the Arctic Ocean.
As you can see in this image, these oceans are connected, forming one large ocean. Water from one of the four oceans is free to move into another ocean. Whales, fish, and other lifeforms from one ocean are free to travel to another.
In addition to these four oceans, a number of seas, bays and gulfs have also been designated. These are generally just smaller portions of one of the larger oceans, though in some cases, such as the Mediterranean Sea, they are almost completely cut off.