Ninety-seven percent of all water on Earth is contained in its oceans. The present distribution of ocean basins on the planet is generally attributed to continental drift. Canada is bounded by three oceans: the Atlantic (82 million sq. km), the Pacific (708 million sq. km) and the Arctic (13.6 million sq. km). These giant bodies of water are interconnected yet each has its own distinguishing features. These features include currents, water masses, sub-marine ridges and definable land boundaries. The ocean currents are a result of prevailing winds and water density differences ("thermohaline") and are complicated by interactions with the continents. The water masses within the oceans retain characteristic salinities, temperatures, densities and trace element composition over large areas. The topography, slope, depth below sea level and composition of the ocean floor are also features that define an ocean.

Oceans play a huge role in defining Canada, a fact expressed in our national motto: A Mari usque ad Mare, which means "From Sea to Sea." With 243 000 km of coastline along the Atlantic, Arctic and Pacific oceans (and an additional 9500 km along the Great Lakes) Canada has the longest coastline in the world. Canada's National Marine Conservation Areas System Plan (Parks Canada, 1995) divides the country's coasts into 29 distinct marine regions. The long-term goal is to have a national marine conservation area within each of these regions in order to ensure the protection of our valuable ocean heritage.