I'm still missing a buried chest, and I'm all out of treasure maps. None of the bartenders will sell me information anymore, and I've already cleaned out every single location in the game.
Also, my challenge menu says I still need to harpoon and kill one more animal. I've killed a bull shark, hammerhead, great white, orca, humpback, and white whale. (That makes six, interestingly enough).
The last Treasure Map can be obtained by sending out your Fleet to Mediterranean Sea and complete the 'Great Reputation II' mission. The harpooning thing seems to be a glitch, many of us are stuck at 5/6.