This will be the first trophy you unlock if you play campaign. (If you play the co-op mode you will only be able to unlock those trophies.)
This is story-related and cannot be missed. After walking through the pirate camp with your brother, something bad will happen, and the pirate "leader" will scream at you and tell you to run. Start hauling cheeks through the jungle and when you fall into the river, you will pop the trophy.