I think marine creature has equal concentration of salty water inside and outside body or they release some chemical substance inside their body to cope with it.So, neither any endo and exosmosis occour.

The approximately 22,000 species of fishes alive today live in virtually all sorts of marine and aquatic habitats that are not unduly toxic. Some, including salmon, lampreys, shad, sturgeon and striped bass, move between freshwater bodies and the ocean at least once in their lives to spawn. Many of these anadromous species do so annually, finding conditions needed for reproduction in one realm and those needed for feeding and growth in the other.

These fishes have to switch over their salt balance physiology when they move from fresh to saltwater and back again. They typically make these adjustments in a brackish estuarine environment--which lies on the way between salt water and freshwater habitats.

for further study

https://www.scientificamerican.com/article/why-do-some-fish-normally/