Is the intercoastal waterway man made or all ready formed there?

Both. A number of barrier islands exist off the Atlantic and Gulf coasts of the United States. The longest of which is Long Island in New York. Some peninsulas, such as Cape Cod, join those barrier islands. There are canals through some of those peninsulas such as the Cape Cod Canal. This was designed to enable boats to sail from Maine to South Texas without ever needing to go out into the Atlantic Ocean or Gulf of Mexico. Some parts of the intercostal have never been completed.