A comprehensive threat detection system is needed to protect critical assets and infrastructure in the nation's harbors. Such a system must necessarily rely on a variety of sensor technologies to provide protection against airborne, submerged, and surface threats. Although many threats can be detected with current technology, which include sonar, radar, visible light, and infrared sensors, there is a substantive gap in harbor defense against quiet surface intruders such as swimmers. This threat cannot be reliably detected with current sensors. Waves and chop occlude visual detection and render sonar blind to relatively small surface objects. The dark of night is also sufficient to defeat visible light detection methods. Wetsuit materials are available that minimize the infrared signature, matching the surrounding water temperature while cloaking the body's heat. However, a range-gated lidar sensor can be used to detect the signature of the swimmer's shadow, which appears impossible to conceal because it depends only on the opaqueness of the swimmer’s body to visible light. A spatially diffuse laser pulse of short duration is used to illuminate an area of interest. The photons are forward scattered in the water, and effectively illuminate the water to an appreciable depth. By range-gated imaging of the water column beneath the swimmer, the absence of backscattered photons is manifested as a shadow in the sensor image and easily detected with existing image processing algorithms. Lidar technology can therefore close the sensor gap, complementing existing systems and providing greater security coverage. The technology has been successfully demonstrated in the detection of moored and floating sea mines, and is readily scaled to a harbor defense system consisting of a network of imaging lidars.