For this achievement you have to fish up a piece of treacherous loot and then hand it into The Hunters Call. These pieces of loot cannot be stored and act like chests or skulls thus requiring you to carry them to hand them in at a Hunter's Call NPC.
The treacherous loot can consist of a number of different things of which I currently think are random. Some examples I have come across are a number of different styles of skulls, one with barnacles on called an 'Old Skull' and one with a hat. The loot can also appear in the form of things like an Old Boot.
Current confirmed list is:
3) Fish Bones
Planks can be caught but they are simply a normal plank.
Will update this guide when I've gained more information.