A variety of flags had been designed for naval battles at some stage in the Civil War; these protected distinguishing marks for ships

https://jeffreypmty481.lakeviewbrief.com/posts/exploring-the-history-of-civil-war-flags-a-journey-through-time-2

A form of flags were designed for naval battles for the time of the Civil War; those incorporated distinguishing marks for ships engaged in struggle on rivers and seas alike.