Most marine organisms have the same body temperature and salinity as the surrounding environment. Why?

The great majority of marine organisms are cod blooded, or ectothermic, having an internal temperature that stays very close to that of their surroundings. A few complex animals-mammals and birds and some of the larger, faster fish-are warm blooded, or endothermic, meaning that they have a stable, high internal temperature.
Because of osmosis, organisms must be isotonic with respect to salinity or water will move across cell membranes until the salt concentration on either side is the same. That could either cause cells to swell and burst or dehydrate and collapse unless counteracted.