The wharf is located at the northern tip of the western island. It serves as one of the East Moeris Trireme’s stops. A well-placed ballista on the dock gives you the opportunity to take down its crew upon arrival.
Sneak around to the base’s north side and slip into the tall grass. From there, slide through the flap on the back of the long tent and collect the treasure on the left. The captain spends part of the day working inside the tent, so watch out for him. If you are lucky, you can catch him signing documents at his desk.