The same ecological and evolutionary processes operate in marine and terrestrial environments, and yet ocean life survives in a dramatically different fluid environment. The ocean is, in effect, a 1.3 sextillion liter water bath with muted thermal variation through time and space, limited oxygen, and intense convective and conductive processes. In this talk, I will trace some of the consequences for evolution, physiology, population dynamics, community assembly, and conservation at sea, including striking contrasts and similarities to patterns on land. I will present evidence that marine animals have evolved narrower thermal tolerances and live closer to their upper thermal maxima than species on land. I will also show that marine species have responded faster and often more predictably to temperature change and temperature trends, across time-scales from seasons to decades. Finally, I will discuss the surprisingly similar histories of extinction in the ocean and on land, and what we can learn from the longer history of human impacts on land. These differences imply the need for distinct conservation and climate adaptation approaches in the ocean.