Assassin's Creed Rogue

I recommend attempting to loot every convoy as soon as you see them. Convoys are ships marked with a £ icon above it. Convoys will spawn randomly in open waters in the North Atlantic however if you are having trouble finding them go to a tavern talk to the bartenders in St John's, Percé or Halifax, which will cost you £200 each time they give you intel. If none of them have intel just wait a little longer and they might. I personally found that looting a convoy and then sailing instead of fast travelling to the next port with a tavern gave ample time for intel to become available and also gave the chance to come across one naturally.