The Treasure Box is the third quest given by The Collector.
Before: I'm interested in a treasure box. It's hidden somewhere in the ocean. Bring me it, and I'll give you some of what's inside it. Sounds fair?
During: What's taking you so long?
Bring The Collector a Treasure Box.
Note - a Treasure Box can be found in the background during the first wave of enemies in Bajo el Mar. It is also dropped by enemies in this zone, or by the Pirate Captain.