This paper describes the first known man-packable unmanned surface vehicle (USV) system designed for radiation localization, either for nuclear treaty verification or sampling after a nuclear incident. Currently, outdoor radiation sampling is done manually or by ground or aerial unmanned systems. An unmanned surface vehicle can operate in shallow nuclear facility cooling ponds and connected streams, rivers, and bays to sample without entering the facility and while maintaining network connectivity to report readings. The Challenge 1 USV is a Lutra 1.1 autonomous airboat modified to carry an UltraRadiacTM. The debris-free area to survey is determined by teleoperation, then the system autonomously generates and executes a novel spiral path and sampling policy. Challenge 1 was tested for proof-of-concept at Texas A&M Engineering Extension Service's Disaster City® with a cesium-137 source placed near one shore of a pond 72 meters from the deployment point. During this test, 775 radiation readings were taken over an area of 57.61 square meters in 18 minutes. These data were used by an inverse distance weighting interpolation algorithm to produce a visual heatmap.