The concept of an easy to use and easy to deploy ocean acoustic tomographic OAT system is presented. The system is composed of a network of buoys and a data inversion online processor. This study concerns the individual node of that network—the acoustic-oceanographic buoy AOB —the data inversion technique and the testing of the system at sea. The AOB is a lightweight surface buoy with a vertical array of acoustic and temperature sensors to be hand deployed in a free-drifting configuration from a small boat. The data are locally stored and transmitted online to a remote station for processing and monitoring. Data inversion is based on a broadband matched-field tomography technique where known and unknown parameters are simultaneously searched for focalization . In situ recorded temperature data serve for algorithm initialization and calibration. The AOB was successfully deployed in several consecutive days during two rapid environmental assessment sea trials in 2003 Mediterranean and 2004 Atlantic . Data collected at sea also show that the AOB can be reconfigured as a receiving array for underwater coherent communications in the band up to 15 kHz.