Ahoy Maties! There has been some confusion as to what counts and what does not count specifically for this achievement in regards to shipwrecked chests.
I made the below video to discuss the achievement and show exactly what counts and what doesn’t. Essentially any chest found within a shipwreck will count. Chests found anywhere else (washed up, skelly ship) will not count (even when it says shipwrecked chest sold).
***Update1.4.4 (3/6/19) now allows chests from skelly ships and washed ashore to count towards this achievement***
***Update (1/23/21). With the addition of Reapers Hideout many have mention you MUST sell these to the gold hoarders and not Reapers. This achievement is tied to the commendation located in the gold hoarders section of your commendations***
***Update (11/1/22). Season 7 has brought us the Sovereign faction. As long as you are on a captaincy ship you can sell items to the Sovereign and it will still count for your progress on this and other faction related commendations/achievements!***
I hope you find it helpful!