Does anyone know? On Android if that's applicable. I've got 6 left, not sure if I've unlocked any already.
Just found one:
Swashbuckler (3,000 pts)
Defeat 5 expert computer players in a single game of the Skull & Crossbones map or the Blackbeard's Wrath map
Global Gamer (1,000 pts)
Win 25 "Play Online" multiplayer games
The description is wrong though, you actually need to win 35 online games.
International Champion (2,000pts)
Win 100 Play Online multiplayer games
I've only got 1 secret achievement left, I'm not sure what some of the other ones are that I've already got.
Actually that last one should be 3,000pts
Click your name on the Home screen, then the Achievements button below: