Some organisms' internal salt levels mimic the external, thus they are able to survive in a salty environment, without water loss. Some have gills and renal or kidney organs, to help concentrate and excrete salt. Fish living in a marine environment will always be in peril of losing water to the environment, as outward osmosis will occur, if not for their gills and kidneys helping to maintain homeostasis. Sometimes, marine creatures, like whales, eat smaller animals like fish or other sea creatures and obtain fluids in that manner, rather than trying to drink seawater which can dehydrate them. Some organisms living in estuaries like mangrove trees and blue crabs are adapted to conditions of salinity constantly changing with the tides. Organisms that can tolerate rapid changes in salinity are called euryhaline. These are the exceptions as most are stenohaline in other environments and cannot tolerate these changes. Cordgrass in salt marshes, has filters on its roots to remove salt from water it absorbs and it expels salt through its leaves. When low tide occurs, bivalves like clams can close up their shells in low salinity water and switch to anaerobic respiration. When high tide returns and salinity increases as well as oxygen, they open their shells and return to feeding and aerobic respiration. These are a few examples of adapting to a salty environment.
It is due to adaptation, some organisms have been in salt water for so long that they have developed mechanisms that helps them survive. While other organisms have been out of salt water so long that they only developed mechanisms that would help them survive in their current environment but not salt water.
SOME ORGANISMS ARE ABLE TO SURVIVE IN SALTY WATER BECAUSE OF SOME ADAPTATIONS EXAMPLE AMOEBA CAN LIVE THAT LIVES IN SALTY WATERS ARE HYPOTONIC TO THE WATER BUT USES STRUCTURES KNOWN AS SALT GLANDS TO KEEP THEMSLVES AS CLOSE TO THE HYPERTONIC NATURE OF THE SALTY WATER AS CLOSE AS POSSILE SO LIVING IN CERTAIN ENVIRONMENT REQUIRE ADAPTATION TO THAT PARTICULAR ENVIRONMENT
Over the course of millions of years, marine life has been evolving to adapt to the high salt content, temperature and light conditions of the oceans. These adaptations depend on the type of marine life. Animals such mollusks, have hard outer shells that protect them from predators, high concentrations of salt, and water erosion. Keen eyesight is an important adaptation for an octopus. In order to catch food in the dark and deep parts of the ocean, they must be able to spot food quickly. Marine mammals also have special adaptations for survival in saltwater. Mammals in the oceans are warm-blooded; their body temperature is regulated to be higher than the temperature of the ocean water. In addition, seals and manatees must decrease the amount of energy and breathing capacity used, in order to dive deep in the ocean. Lastly, reptiles that have evolved to live in saltwater, such as the sea turtle, have harder shells and feet designed for swimming. These organisms are found in tropical and temperate oceans, but there are also organisms that can survive even the harshest of saltwater environments. Halophiles, small microbes, can live in environments up to 10 times more saline (salty) then the oceans. The Great Salt Lake, in Salt Lake City, Utah, is home to these organisms. The salt content in the Great Lake is so high that you can actually walk on salt flats. Understandably, these microbes have very specific adaptations. I hope this helps!