Lots of good instrumentals produced by
Audiomachine and
Two Steps from H*ll. There are portions of their scores which are too electronic for a pirate-style game in my opinion, but I find they're nice sources of inspiration. I'll list a few of my favorites because it's hard to choose only one.
Audiomachine:
"Black Cauldron" - (I enjoy the tension of the opening -- could be a nice boss introduction.)
"Sands of Time" - (Again, the strings in the intro seems fitting. Not sure about the choir fitting into theme, but perhaps it could fit as an SvS or PvP song.)
Two Steps From H*ll:
"All the King's Horses" - (Fast tempo -- another SvS track, or perhaps in the future, a background track for another Black-Pearl-esque boss fight. Just spit-balling here.)
"Dark Ages" - (Extremely brassy, heavy, dark intro -- I see boss fight into immediately, but that's just a first glance.)
Again, there are many other solid scores composed by these groups but I tried to keep the list short and to the point. Trust me, I could talk for days on this stuff. Thanks!