Fuel supply system for boats fuelled by alternative fuels and relative boats

Fuel supply system (4) for motors (8) of motor boats (12) both of the outboard and inboard type, fuelled by alternative fuels. The system comprises at least one pressure regulator (24) which receives the fuel at a supply pressure and delivers it in output at a delivery pressure, different from the supply pressure, to injection and/or solenoid valve devices which supply the motor (8) with fuel. The system (4) comprises at least one closed container (40) which identifies an inner volume (44) containing said pressure regulator (24) and, at least partially, said supply (28) and delivery (32) ducts, the closed container (40) hermetically tight sealed to the gaseous fuel and comprising at least one vent (48) in fluidic communication with the inner volume (44) of the container (40). The vent (48) fluidically connecting the inner volume (44) with the environment outside the system (4) so as to channel any fuel leaks towards the outside of the boat (12).