Igor says this is a good question mate. Igor always seam to have so much to do and not enough time to do it on the high seas. Igor think this forum is a great place for Mastered pirates to find things to do!! Igor say here you can find events or clubs to join. Igor say and if there isn't something going on that you like you can start your own event/meeting or club!!
Aside from looting, leveling smaller pirates, helping others, and so forth, I keep myself entertained in game mainly through playing and hanging out with my friends and guildmates. Group and guild events, parties, etc. etc. are the kind of things I try to keep frequent in my guild, because the majority of our members do have maxed out pirates.
So, it's not like the game is completely unplayable with a maxed out pirate - you just need a little creativity, and a good group of compadres.