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

https://ericksigh824.birchreport.com/posts/exploring-the-history-of-civil-war-flags-a-journey-through-time-7

A style of flags have been designed for naval battles all through the Civil War; these blanketed distinguishing marks for ships engaged in fight on rivers and seas alike.