Why does crocodiles and alligators does not live in the sea?

There are three related species - crocodile, alligator and ghariyal. Crocodiles have active lingual (on tongue) glands that can secrete salt - these glands are present also in alligators but aren't active. That's why crocodiles can live in ocean where alligators can't survive for long (ghariyal is a purely freshwater species with no such gland at all).

Another member of the family is the caiman, which is an alligator like group found in South America.

Though the crocodiles are poor swimmers, the larget living reptile called estuarine crocodile (Crocodylus porosus) can ride sea wave and get across the sea from one to another nearby land and live in mangrove forest or estuary near the seashore.

(source : sciencenews.com; June 7, 2010).