No one is sure exactly when the geographical study of the ocean began, but it surely began with the earliest sailors embarking out upon its face. Learning the patterns of ocean currents was absolutely vital for early fishermen, traders and adventurers.
During the Egyptian Bronze Age, neither the Egyptians nor the Greeks were accomplished seafarers. The Phoenicians, however, had an extensive knowledge of the currents and tides of the ocean that allowed them to venture far and wide and make settlements throughout the Mediterranean Sea. They also ventured out into the Atlantic Ocean.
Aristotle was the first to publish scientific works regarding the ocean. His writings discussed the bathymetry (depth) of various bodies of water, the gradual shift of seas and continents and the biology of various plants and animals in the seas. One important observation of note is that he recognized that whales and dolphins are mammals rather than fish.
Other than the discovery of faraway seas and their attendant currents, little work was done in the field of oceanology for many centuries after the time of Aristotle. In the 16th century, though, "amateur naturalists" began to arise. These were educated, well-endowed men who studied various aspects of the natural world as a hobby. The explorer James Cook was one of these men.
Many credit Edward Forbes as the father of modern oceanology. Following in his footsteps, Charles Darwin studied sea biology as well as the birds and land mammals for which he is most famous.
In the 19th and 20th centuries, various governments began to commission sea vessels to go on oceanological voyages to study marine life in various parts of the world. The first of these was the H.M.S. Challenger. The U.S.S. Albatross is also notable in that its expedition was the first to employ extensive dredging of the sea floor to get biological specimens and soil samples.