Fathom is an advanced airborne lidar system optimized for high resolution shallow water bathymetry and topographic mapping in coastal zones, rivers, and inland waterways. It is specifically designed to support river management, sediment transport studies, environmental monitoring, coastal security, and geomorphologic investigations, where shallow depths and complex land–water transitions dominate. The system employs an integrated topo bathy architecture combining a dual frequency 60 kHz bathymetric laser (532 nm and 1064 nm) with a four channel segmented detector array optimized for shallow water. This configuration enables robust bottom detection up to 21 m in clear water (Kd ≈ 0.1) for coastal and riverine environments, while maintaining deeper water capability up to 32 m. An effective 240 kHz bathymetric sounding rate in full waveform mode is complemented by a 1.5 MHz topographic channel, allowing seamless, single pass acquisition across land and water surfaces under a unified calibration framework. This paper presents validation results from test flights over Lake Ontario and Lake Huron, using Canadian Hydrographic Service (CHS) reference data. Results demonstrate sub decimeter vertical coherence, reliable water surface detection, and shallow water performance consistent with system design expectations. The measured accuracies meet IHO S 44 Order 1a bathymetric and USACE QL0B topographic standards, confirming Fathom’s suitability for precision shallow water mapping, hydrographic charting, and rapid response operations in dynamic coastal and riverine settings.