Clean Sea (Continuous Long-term Environmental and Asset iNtegrity monitoring at SEA) project [Gasparoni et al., 2013a] concerns the use of commercially AUV [Gasparoni et al., 2013b] with a modular interchangeable payload for environmental monitoring and inspection tasks around Eni oil & gas offshore infrastructures. The platform identified for the development of the project was SAAB Seaeye Sabertooth. The project defined peculiar hardware and software architecture, where the payload controller is separate from the AUV controller and able of modifying “in real time” the mission strategy [Buffagni et al., 2014]. The environmental payload chosen for AUV was: Temperature [°C], Salinity [psu], Methane [μmol/l], Colored Dissolved Organic Matter [ppb], Chlorophyll [μg/l], Turbidity [m-1 sr-1], Oxygen [μmol], Oxygen [%], Polycyclic Aromatic Hydrocarbons [μg/l], pH, Oxidation Reduction Potential [mV]. The Lake Vättern (Figure 1), the sixth largest lake in Europe and the second largest lake in Sweden after Lake Vänern, has been chosen as test field for the first AUV ascertainment. In detail, 36 tests have been realized in two different sites (Figure 1), located in the North-East (latitude 58°32’32.27”N, longitude 14°58’9.03”E) and North-West (latitude 58°32’32.27”N, longitude 14°58’9.03”E) side of the Lake, from November, 2012 to July 2013.