A device for converting wave-motion power, comprising: - a float (5, 50), which, in use, is movable in opposition to the gravity acceleration in response to the wave-motion; - an input shaft and an output shaft rotatable about respective axes (18,3); - first transmission means (6,13,15,16,17) for transmitting the motion of said float in a reciprocating rotary motion of said input shaft; - second transmission means, which transfer torque from said input shaft to said output shaft, are configured so as to split the torque in two torque paths (21,22) and comprise two freewheels (23,24) arranged respectively along said paths, (21,22); said freewheels (23,24) being configured so as to make the rotation of said output shaft unidirectional, independently of the rotation direction of said input shaft, said device being characterized in that said float (5, 50) has an overall mass measured in kilograms comprised in the range between 25% and 50% of the volume of the float measured in cubic decimeters, said overall mass being evaluated taking into account at least said first transmission means (6,13,15,16,17), wherein said float has a non-uniform density and comprises a first portion having a first density and a second portion having a second density lower than that first density, said first transmission means comprising an arm hinged to said float from the opposite side of said first portion with respect to said second portion, wherein said second portion has a maximum transverse dimension of at least four times the maximum transverse dimension of said first portion, the mass of said float can be considered concentrated in the volume of said first portion and wherein said first portion has a maximum height of at least 1.5 times the maximum height of said second portion. - a float (5, 50), which, in use, is movable in opposition to the gravity acceleration in response to the wave-motion; - an input shaft and an output shaft rotatable about respective axes (18,3); - first transmission means (6,13,15,16,17) for transmitting the motion of said float in a reciprocating rotary motion of said input shaft; - second transmission means, which transfer torque from said input shaft to said output shaft, are configured so as to split the torque in two torque paths (21,22) and comprise two freewheels (23,24) arranged respectively along said paths, (21,22); said freewheels (23,24) being configured so as to make the rotation of said output shaft unidirectional, independently of the rotation direction of said input shaft, said device being characterized in that said float (5, 50) has an overall mass measured in kilograms comprised in the range between 25% and 50% of the volume of the float measured in cubic decimeters, said overall mass being evaluated taking into account at least said first transmission means (6,13,15,16,17), wherein said float has a non-uniform density and comprises a first portion having a first density and a second portion having a second density lower than that first density, said first transmission means comprising an arm hinged to said float from the opposite side of said first portion with respect to said second portion, wherein said second portion has a maximum transverse dimension of at least four times the maximum transverse dimension of said first portion, the mass of said float can be considered concentrated in the volume of said first portion and wherein said first portion has a maximum height of at least 1.5 times the maximum height of said second portion.