daTextBox=The places to live in the ocean include the seafloor as well as a much larger vertical space above.  While most terrestrial organisms can't hover around in "airspace" all of the time, many marine organisms spend their entire lives in "waterspace" above the seafloor. 
Marine life is concentrated closest to the sea surface where it's sunny, requiring marine organisms to be physically capable of "hanging around."  But it's a dangerous place with few places to hide, so other places offer advantages too.
Even where it's nearly freezing, totally dark or food is extremely scarce, some organism is suited to making a living at each depth.  Explore the key environmental conditions that describe the challenges of the ocean depths.