The lidar overlap function is defined as the fraction of the transmitted beam that is within the receiver field of view. For the case of bistatic oceanographic lidar from the deck of a ship, the overlap function can vary from pulse to pulse under the influence of the rough sea surface. This paper considers the overlap function as a function of depth for a bistatic lidar operating from the deck of a ship. The effect is calculated using a Monte-Carlo approach, with a Pierson–Moskowitz spectrum of surface roughness and optical ray tracing through that surface. The results show that a significant decrease in the overlap can result, even at low wind speeds.