The TrueAchievements description is a little confusing for this achievement. The description on the Xbox says "Cash in a stolen Captain's chest."
This should be clear as to what is expected of you.
However, I have stolen Captain's chests several times from sloops and galleons alike and the achievement hasn't unlocked. At the time of writing no one has unlocked the achievement, thus it must still be broken.Update 24th April 18:
According to the official patch notes for patch 1.0.5 this achievement has now been fixed. Apparently this will not unlock retroactively.https://www.seaofthieves.com/forum/topic/55501/patch-notes-1...Advice for the achievement:
You probably shouldn't attempt this solo, although it's possible you should team up to improve you odds of boarding, killing and running away with a Captain's chest. Additionally, you don't have to be the one to cash the chest in, your crew mate can steal and cash in the chest and everyone in the crew should unlock the achievement! (Thanks to Doodler
Stay close to an outpost. Many times I have played with crews that want to cash in a Captain's chest straight away due to its value. So, staying close to an outpost and waiting for other ships to come close is a good way to improve the odds.
If you steal a chest at the outpost, it might be safer to retreat back to your ship and sail away. Don't just rush to try and cash the chest in, you might be outnumbered and you will run slower with the chest in hand. You could sail to another outpost and drop it off there.
Any other advice? Let me know, or see the comments. Good luck!
Update: I have raised a support ticket on the SoT website to hopefully get this fixed. I'm not sure if the ticket is publicly accessible, however the URL is: https://support.seaofthieves.com/hc/en-gb/requests/50855 fingers crossed!