We present the in situ use of an underwater mass spectrometer (UWMS) for the detection and quantification of active submarine methane seeps. The key benefit of using this novel technique is the rapid instrumental response, which allows an up to 750 times higher sampling frequency of dissolved gas concentrations compared to other methods. Among other major and trace gases, the increased data density and accuracy allows for the improved determination of methane point-sources. Methane is the most abundant organic compound in the atmosphere and its influence on global climate is the subject of current scientific discussion. One source of atmospheric methane with variable strength is the ocean’s seafloor e.g. through the destabilization of gas hydrates. These submarine methane sources are characterized by rising gas bubbles and/or the diffusive flux of gas into the water column. While ex situ sampling methods are limited by the time and costs, the primary limitation is the difficulty in capturing concentrations that are highly dynamic in time and space. Using an optimized UWMS (significantly improved in detection limit by a cryo-trap system) in the North Sea (Atlantic Ocean), we have obtained high-resolution 3D distribution patterns of dissolved methane in the water column. In addition to presenting novel methodologies using in situ mass spectrometry to detect, map and calculate the methane inventory above submarine sources, we will present a new configuration of instruments to determine the diffusive transportation rates at the sediment-water-transition-zone.