The fundamental difference between aquatic and marine life is that aquatic life occurs in any body of water, whereas marine life specifically lives in saltwater environments. Marine organisms often have special adaptations that allow them to thrive in this particular type of aquatic environment.
Aquatic ecosystems - and aquatic life - is conventionally divided into two basic forms: freshwater systems and saltwater systems, with the latter also commonly known as marine systems. About 72% of the Earth's surface is covered by water, which means that aquatic ecosystems are the most common type of ecosystem on the planet. Almost all of this water (about 71% of surface area and 97% of the water itself) is classified as marine. Marine ecosystems are then further subdivided into the oceans themselves, coastal salt marshes and tidal zones, river estuaries, mangrove forests, coral reefs, the sea floor, and tiny pockets of extremely unusual life forms surrounding deep-sea thermal vents.
Technically, aquatic refers to all water-based life, but it is also commonly used to refer specifically to freshwater life. Both freshwater and saltwater organisms often have adaptations allowing them to function in those particular environments, which make it difficult and extremely energy-consuming for them to thrive in the other. For this reason, although general classes of animals (like fish) can be found in both fresh- and salt-water environments, the species themselves tend to be found only in one or the other. In particular, marine life must find ways to balance internal salt content with a much higher salt content in the surrounding water, although it also benefits from the much greater support that this water gives. (This explains why there are many types of marine life, ranging from small invertebrates up to large cetaceans, which lack the necessary bone structure to support their own weight.) Many saltwater species are unable to survive for long periods in fresh water, and many freshwater species are unable to survive in salt water, because they lack the capacity to maintain this balance in an unfamiliar environment.
While aquatic life can occur anywhere, marine life is actually concentrated in a few specific areas of the ocean. Over 90% of marine life, both in terms of numbers of organisms and in terms of overall biodiversity, is believed to occur along coastlines, with only 10% dispersed over the remainder of the seas. The coastal zones are typically shallower and warmer, and include such fragile and precious ecosystems as coral reefs. These structures actually consist of calcium deposits left by massive colonies of coral organisms. Around the world, most coral reefs are in serious and possibly permanent decline, probably as a result of ocean pollution and overfishing.