Skeleton Pirate Ship

[xcv461]

I need to get a key from a Skeleton Pirate Ship,sank at least 30 Skeleton ships but still no key.Where do I find it?
Called a Skeleton Frigate. I thought they were all Frigates.
it says recover a skeleton key by sinking navy ships

I'll just max out my weapons till I can find the key

Finally got my key,just left Tortuga and he was right in front of me.Had 3 other ships hitting the Skeleton Ship and it waited till I got off a shot before it went down.That was by far the hardest quest so far.