Water is 800 times denser than air [...] The cheetah, for example, runs faster than many fish swim, yet has far less streamlining by comparaison.Gravity, p 247-248
[...] aquatic organisms can exist in a nearly "weightless" environment. While land organisms must have a rigid structure to resist gravity, some aquatic organisms such as the jellyfish grow quite large with virtually no rigid structure at all. This makes it possible for some aquatic organisms to live their entire life [...] never or rarely touching the bottom [...] even birds spend a significant amount of theirs lives perched.Light, p 248[...] no 225 kg terrestrial animal can hover in air at all.
Aquatic organisms feel and respond to gravity by fish swimming "right side up." In try weightlessness, no reference to up, or down exists.
In the clearest water, light cannot reach deeper than about 180 metres [...] Consequently, plant life survives only at the shallower depths [...]Oxygen, p 248
Proportionnally, little oxygen exists in water, requiring aquatic organisms to have gills, which compared to terrestrial lungs are significantly more efficient at gathering oxygen.
On land, plants dominate. [...] Long-lived plant communities control short-lived animal communities. Terrestrial ecosystems tend to be described in plant-terms: redwood forest, grass prairies, rain forest, etc.[...] Animal populations dominate many aquatic ecosystems, which are referred to in animal-terms: coral reef, oyster beds, clam beds, etc.