## 1 Introduction  
The adoption of autonomous systems and mobile robots has witnessed a steady growth in several fields such as industrial automation, inspection of structures, the space industry, as well as healthcare and domestic services. Typically, the choice of robotized solutions is motivated by the need to tackle challenging or dangerous tasks in hazardous environments such as the inspection of nuclear plants [1] or the detection and disposal of explosive devices for civil or military operations [2] but it is also dictated by the demand of accuracy, repeatability and efficiency in tasks that can be highly time consuming, costly, and labor intensive such as pipe-lining, data collection, model digitizing, together with anomaly and defect detection in structures [3, 4]. Lately, the research has been geared also to the development of marine robotic systems, as some aquatic operations are very difficult for human capabilities or unsafe. A big effort has been aimed at proposing new designs of underwater autonomous vehicles [5], also exploiting bio-inspired principles, such as octopus [6], cephalopod [7], and turtle [8] inspired robots. Sectors like oceanographic science, hydrography mapping, sea exploration, and marine archeology get great benefit from the adoption of unmanned marine robotized vehicles allowing difficult tasks to be tackled like seabed inspection or search and rescue operations [9; 10; 11; 12; 13]. In particular, the study in the latter field has been motivated by the high number of sea accidents globally recorded: with around 372,000 deaths counted every year [14], drowning still represents the second highest cause of unintentional death in the world and the 20% of surviving people suffer severe and permanent brain damage, mostly because the rescue occurs too late to keep brain functions unaltered. Moreover, the data show that a great part of trained rescuers or would be rescuers lose their lives or suffer serious injuries trying to save other people, thus there arises a need for solutions to support rescue operations. The main solutions present in literature are classified in aerial, surface, and underwater categories. Aerial solutions are designed to work in bad weather conditions and adopt a lifebuoy or a stretcher hung from the vehicle to aid people in distress [15]. A new perspective is given by the adoption of ekranoplans, vehicles able to fly at only a few meters above the water surface, thanks to the exploitation of the ground effect [16]. Surface rescue vehicles are strongly limited by the poor capability of handling in rough sea conditions. In this category, a solution that is experiencing particular improvement and can be effective in aquatic rescue is represented by personal watercrafts [17; 18]. Underwater systems have been presented in the last decades but their spread is still limited due to high costs, maintenance, size, and weight [19; 20; 21]. All these solutions need a crew to perform the recovery operations and are effective only for conscious people. This paper aims to fill the lack of literature for autonomous marine rescue vehicles presenting a patented device [22] designed to save conscious and unconscious people in distress in the water without the help of a crew and with the capability of maneuver and operations even in the case of a rough sea. The proposed robot is based on an electric powered tele-operated device in the shape of a stretcher that can be controlled from a maximum distance of 400 m from the shore or directly from boats and helicopters if the drowning is occurring in open sea. The solution to tackle rough sea conditions is to move the robot underwater to minimize the effects of the waves on the stability of the vehicle. The choice is motivated also by the statistics stating that most rescuing is performed when the victim is already under the water surface.