Recently, due to the deployment flexibility of unmanned aerial vehicles (UAVs), UAV-assisted mobile relay commu-nication system has been widely used in the maritime communication. However, the per-formance of UAV-assisted mobile relay com-munication system is limited by the capacity of wireless backhaul link between base station and UAV. In this paper, we consider a caching UAV-assisted decode-and-forward relay com-munication system in a downlink maritime communication. For the general case with multiple users, the optimal placement of UAV is obtained by solving the average achiev-able rate maximization problem through the one-dimensional linear search. For a special case with single user, we derive a semi closed-form expression of the optimal placement of UAV. Simulation results confirm the accuracy of analytical results and show that the optimal placement of UAV and the average achievable rate significantly depend on the cache capacity at UAV. We also show the difference between the performances of the air-to-ground model and the air-to-sea model.