## 1 Introduction  
A comprehensive understanding of the processes and conditions that affect the state of the marine environment is a key parameter in many fields of environmental science and engineering as well as in natural hazard forecasting efforts. Climate conditions and variations, floods and prolonged seasons of high temperatures and droughts, water pollution prevention and sustainable management of marine resources, renewable ocean energy and maritime transport are topics of high social and scientific impact that relies on deep understanding of the oceans functionality. This understanding is strongly dependent on observations and monitoring of key parameters at a wide range of space and time scales. For this purpose, technologically advanced solutions are steadily emerging such as stationary and mobile observing platforms, new sensors and auxiliary communication modules for smart and real-time monitoring of essential seawater parameters. The increase of technological and engineering efforts towards the integration of innovative sensors into existing multi-parametric observing systems was also favored by the Marine Strategy Framework Directive (MSFD) [1] that introduced new parameters to describe the Good Environmental Status of the seawater. Specifically, human-induced marine underwater noise was defined a pollutant and its monitoring was considered essential to assess its effects on marine life; similar attention was paid to increase the monitoring of chemical and radioactive elements and compounds that potentially could be contaminants. Until now, existing acoustic surveys have been typically oriented to study sea mammal's behavior and protection, although underwater sounds could be of interest to monitor rainfall and wind over the sea surface as well as shipping noise. Few observatories, mostly cabled, have been equipped with hydrophones to listen to the ocean in a continuous way (e.g., [2; 3; 4]), but several experiments of limited duration have been carried out using ARGO floats [5] or fixed stations [6; 7]. On the other hand, the use of autonomous devices able to monitor chemical compounds is still missing due to the considerable power requirements to perform measurements based on analytical methods. Some steps forward have been made as regards the monitoring of radioactivity through calibrated measurements for studying oceanic processes [8]. In the marine environment, in situ measurements of radon progenies and potassium (\({}^{40}\)K) together with underwater sound records have been utilized as tracers of submarine groundwater discharge [9] and rainfall in the open sea [10]. Also plenty of measurements of artificial radioactivity have been occurred after the Chernobyl accident [11] for radiological surveillance and monitoring purposes. Although autonomous sensors able to monitor underwater sound and marine radioactivity can be complementary each other to study rainfall or to detect groundwater discharges, their integration into oceanic platforms is not so conventional due to the intrinsic difference of their operating mode, quality control and quality assurance procedures, heterogeneity of data stream, data processing algorithms, and last but not least due to the power required for continuous and long-term operation. In this frame, the main technical challenges consist in: (a) downsizing the sensors, (b) increasing the channels of serial outputs function of the electronics onboard the observing systems that have the role to schedule the sampling scheme and (c) guaranteeing enough power supply to the scientific payload to be continuously operational. Among the platforms operational at sea, the Eulerian observatories are the most useful to integrate such kind of instruments, since they allow measurements with a multi-annual temporal scale of operation also in harsh environments (i.e., deep sea, polar regions, highly trafficked basins, etc.) [12].