A number of flags had been designed for naval battles for the period of the Civil War; these included distinguishing marks for ships

https://damienqulw756.laurelcurrent.com/posts/a-closer-look-at-the-most-iconic-civil-war-flags-of-the-era

A variety of flags were designed for naval battles right through the Civil War; those included distinguishing marks for ships engaged in strive against on rivers and seas alike.