I have to admit, I did not think of shanties. That is a genius idea in its simplicity and would add much more of a flow to the game. By flow I mean the game would seem to progress faster. Assassins Creed 4 shanties were a major part of why I enjoyed the open seas aspect, I would suggest getting some of the more popular ones from AC4 if they are not copy-write.
May I also add to this that perhaps weather changes in battle to also add to the ambient environment of combat? Not the battle as a whole, but perhaps an area has the possibility of storms from clouds, or even cloud changes. Ambient changes in environment can have a huge impact on gameplay, POTBS would have benefitted greatly on its OS mechanics with this concept in some form.