Why do some organisms survive in salt water but not others?

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!