The sea singer, using still water performance, could reduce the DC 20 sailing check made during ship-to-ship combat (using the rules on the Skull & Shackles Player's Guide)?
I am not sure if the "calm rogue waters" is a desciptive part or not.
After all the reduction of the DC is could far more of what usually the "rough water" situation add (e.g. rough water add only +2 to the DC of an achrobatic check but still water could reduce)