The quality of water and air in enclosed waterways, such as harbors, channels, lagoons, et cetera, is menaced by the pollutant emissions of conventional internal combustion engines (ICEs). It would be highly desirable to enter these areas with boats powered only by electric motors. On the other hand, a pure electric mode requires a very heavy and expensive battery pack, in order to guarantee a suitable range. Hybridization appears as the most straightforward way to meet these conflicting requirements. However, this solution is far from trivial, and it needs a new approach to the design of ICEs, in order to get the maximum advantage from the coupling to the electric motor, and to mitigate the related drawbacks (higher complexity, weight and cost). The same consideration is applicable to the electric system. The paper reviews the design of an innovative hybrid powertrain for medium size leisure yachts and other small boats. The power unit is made up of a new 2-stroke opposed piston (2S-OP) Diesel engine, delivering a maximum brake power of 285 kW at 3200 rpm, and a serially connected electric motor, able to provide a continuous power of 100 kW, weighting just 20 kg. This is a permanent magnet synchronous external rotor, designed to match the electrical parameters of the power inverter (with a DC voltage of about 400 V, weighting 7.5 kg), as well as of the Lithium Ions battery pack. The electric motor is integrated in the gearbox housing and it will share the cooling system of the internal combustion engine (ICE), being able to operate at the same temperatures. Obviously, the electric motor works also as a starter/generator for the ICE. The paper is focused on the design of the 2S-OP engine: CFD 3D simulations are used to design the scavenging and the combustion systems, while CFD-1D analyses (by GT-Power) enable the optimization of the main engine parameters, including the characteristics of the supercharging system. The performance parameters of the 2S-OP engine are then compared to the ones of a state-of-the-art 4-S turbocharged Diesel engine, delivering the same power output. Finally, some qualitative considerations are made about the pollutant emissions of the 2-Stroke engine, and the relative cost of the after-treatment system, in comparison to a standard technology. It is demonstrated that the proposed engine is more efficient, lighter and cleaner than the equivalent 4-S, and it appears as an ideal solution for a hybrid powertrain.