Lagoon

A lagoon is a body of salt water partly separated from the sea. There is usually some sort of barrier which separates and protects the lagoon. This may be a pebble or shingle beach, sand bank, or perhaps a coral reef surrounding an atoll. An atoll is an oceanic island formed by a volcano. 
"Lagoon" refers both to coastal lagoons, and the lagoons in atolls, formed by the growth of coral reefs on slowly eroding central islands.