The ocean is temperature-stratified (thermoclines). The uppermost shell is warmed by the Sun, at least in the tropics.
Just below is a mixed surface layer. Winds and tides mix the water, but the biota there also do their part, including the copious congregations of jellyfish. Even zooplankton are substantial contributors to the mixing. All told, life in the near-surface layer contributes as much to mixing as the wind and waves. This mixing provides for both oxygenation and distribution of warmth to a greater depth.
Below the thermally mixed layer is a narrow thermocline that separates the warm surface water from the colder and heavier water beneath. At the poles, much of the ocean is equally cold.
Though life concentrates near the surface, it is the cold, heavy water in the deep that revitalizes the food web. The marine snow of organic detritus drops down in prodigious volumes. Most marine snow is consumed within the top 1,000 meters, but a considerable quantity reaches the deep. Upwelling cold-water currents return that nutrient-rich supply toward the surface.
The richest concentrations of sea life occur where these cold currents come up. Near the coast, where steady winds sweep warm surface waters offshore, deep-water rises to fill the displacement. In temperate regions, winter storms churn the water, delivering cold comfort to life there.
In the tropics, the separation of warmth at the surface and cold at depth is so great the even hurricanes and typhoons cannot thoroughly mix the two. Tropical seas stay crystal clear, bereft of the microscopic clouds that bring fine dining from below.
On land, oxygen is available at a fairly constant level: 210 milliliters per liter. At sea, usable oxygen enters only at the surface.
Because most of the water in the deep ocean originated on the surface, in or near the polar regions where it sank, it holds the most dissolved oxygen. These down-welled water masses may spend centuries in the deep before rising again. As life is sparse there, oxygen is rarely depleted.
The intermediate depths are where oxygen is at a minimum. In the Pacific Ocean, the oxygen minimum zone (OMZ) is between 500 and 1,000 meters down. Oxygen may be only 1/30th of its concentration on the surface.