## 1 Introduction  
Among various active geophysical techniques, electric and electromagnetic (em) methods are strongly effective for the detection of archaeological features located in the subsoil at different depths and scenarios [1; 2; 3; 4; 5; 6; 7]. Localization of anthropic structures placed in the soil is possible due to the contrast of the em physical properties between the materials constituting the buried objects and the subsoil where they are \"preserved\". The presence of the \"geophysical contrast\" between the analyzed structures and background soil represents the necessary conditions for successful research in the archaeogeophysical field. However, the presence of subsoil with a high-water content (close to the saturation) could be a limit for the research due to em signal attenuation problems and \"homogenization effect\" of the subsoil physical behaviour. From one side, the attenuation phenomena related to a high water and clay content of the soil play a crucial role for the em methodologies and strongly influence the depth of investigation, resolution, and velocity of propagation [8]. From the other side, a homogenous electrical behaviour of the subsoil due to high water content, also in presence of archaeological features, could be a hard problem for the resistivity techniques. Moreover, despite limitations caused by the presence of water, em and electrical geophysical prospecting are often the only way that can support archaeologists to identify interesting areas to excavate. Additionally, archaeogeophysical investigations in underwater conditions today represent one of the most interesting challenges for the geophysical applications [9]. In this fascinating field, the geophysical methods have been fast developing in the last two decades and, regarding the sub-water archaeological surveys, the use of acoustic (sound or sonar) systems in well-established [10]. These include echo-sounders, multi-beam swath systems, side-scan sonars, sub-bottom profilers, and bottom classification systems. Nevertheless, notwithstanding the great depth of investigation, these methods are characterized by a low resolution; therefore, in scenarios with fresh water, great attention is addressed to the investigations of underwater structures with high resolution techniques, among which stands out the ground penetrating radar (GPR) [11; 12; 13], joint to other methodologies such as direct current (DC) electrical resistivity method. The use of GPR in water covered areas is still a challenge because it suffers a series of problems linked to the em signal attenuation in the presence of water. However, it has been successfully used to detect geological structures and archaeological features beneath see, rivers, ponds, and swamps bottom [14; 15; 16; 17; 18]. In general, the surveys were carried out using small boats and antennas with frequencies between 100 and 400 MHz. On the contrary, electrical resistivity methods are more commonly used in water covered areas (stream, river, wetland, lake, and sea), for evaluating subsurface conditions for hydrogeological and environmental purposes [19; 20]. Rarer are applications in the maritime archaeological field [21; 22]. In general, surveys in water-covered areas include conventional surveys using a multi-electrode resistivity system where part of the survey line crosses a river or a lake, and surveys conducted entirely within a water-covered environment. In applications in seas and rivers, this method has been performed both with fixed electrodes placed on the water surface or on the seabed and riverbed [23; 24; 25; 26; 27; 28; 29] or by dragging the electric cable on the water surface with the aid of ships or boats [30; 31]. In the first case, usually, the cable is weighted in order to allow direct contact with the marine sediments.