What fish can adapt to both salt water and freshwater?

Salmon

There are actually several categories of freshwater/saltwater migratory fish.

Wikipedia breaks them up into several categories: Diadromous, Anadromous, Catadromous, Amphidromous.

See links for more information.

Some predators can also hunt in both saltwater and freshwater.

Sea Lions will enter rivers such as the Columbia River during fish migrations.

Bull Sharks are also known to hunt both in saltwater and in freshwater rivers.

Crocodiles have also been seen miles out to sea, scarily..