Laden With Treasure
Uncommon20.4%ObjectiveOptionalAnywhere
You had twenty chests in your crew's possession... at the same time!
Yeah, loot lots before turning in, and pray nobody takes you out. Easiest when working together for the achievement with 4 players on a galleon.
Use the Gold Hoarders' Ancient Vault quests - for 8 or so chests at a time.
Note the chests don't actually have to be on your ship - just in your "possession"; so just picking a chest up counts as making it yours.