A type of flags had been designed for naval battles in the time of the Civil War; those covered distinguishing marks for ships

https://myleslgdi553.summitquill.com/posts/the-craftsmanship-behind-historic-civil-war-flags-techniques-and-materials-used

A variety of flags were designed for naval battles at some point of the Civil War; these covered distinguishing marks for ships engaged in combat on rivers and seas alike.