The term 'body of water cut off from the sea' can be summarized as a LAGOON, which is a six-letter word. A LAGOON is a shallow body of water that is separated from the ocean by a barrier such as a sandbar or coral reef. It is typically found in coastal areas and is often characterized by its calm waters and abundant marine life. LAGOON ecosystems are important for providing habitat for many species of fish, birds, and other wildlife.