It can be found at Brooke's Head Lighthouse, lying on a table at the top in the lighthouse's lookout, next to a red chair.
Caught some of my boys tryin to steal some of the loot we took off that trader. Had to put a bullet in him to show the others. Just in the leg, for now. Put all the stuff in a trunk and dropped it in one of the fishing nets. I'll hang onto the key for now in case they get any ideas.