The smuggling of special nuclear material (SNM) has long been a concern. In April 2009, President Obama declared that a terrorist acquiring a nuclear weapon was the most immediate threat to global security. The Second Line of Defense (SLD) initiative was stood up by the National Nuclear Security Administration to deter, detect, and interdict illicit trafficking of nuclear and radioactive materials across international borders and maritime shipping. The SLD initiative does not provide for the detection of SNM being carried on small, personal watercraft. Previous work examined the possibility of using active neutron detectors to induce fission in SNM and detect the response. This thesis examines the possibility of detecting SNM using passive 3He neutron detectors. Monte Carlo N-Particle (MCNP) simulations were run to determine the best detector configuration. Detecting sources at increasing depths, detecting moving sources and the effects of waves were also simulated in MCNP. Comparisons with experimental measurements showed that detectors parallel to the surface of water were best at detecting neutron sources below the surface. Additionally, stacking detectors and placing a cadmium sheet between the polyethylene blocks resulted in a greater ability to determine the height of a source by taking the ratio of count rates in the lower and upper detectors. Using this configuration, a source of strength 3.39 x 10^5 n/s could be detected to a depth of 12.00 in below the water surface. Count rates in the presence of waves did not average out to count rates taken above a flat plane of water. Detectors closer to the water performed worse than above a flat plane while detectors placed higher recorded more counts than above a flat plane. Moving sources were also simulated; sources under water, 3.00 ft from the detectors, and moving at 5.8 kts could be detected above background.