The term 'BIG BODY OF WATER' can be summarized by the solution SEA, which is a three letter word. SEA is an acronym for 'saltwater, estuarine, and aquatic ecosystems, ' which are large bodies of water that contain saltwater, estuarine, and aquatic ecosystems. These bodies of water are typically found in oceans, seas, gulfs, and bays. They are home to a variety of marine life, including fish, mammals, and invertebrates.