u2032 may slide said float as can be seen having an upper section 102 u2032 and a lower section 202 u2032 which are substantially conical or frustoconical so as to facilitate the upward and downward movement thereof also in the case of sea waves of smaller amplitude underneath this float 2 u2032 the offshore dam 17 has formed therein an expansion chamber 21 for the incoming sea wave which communicates with the section 22 of seawater outside the offshore dam 17 and is provided with a suitable water inlet opening situated above the hydraulic pump 6 as can be seen the hydraulic pump 6 is similar to that shown in the previous embodiment while the duct 11 conveys the water delivered by this pump 6 to the turbine 16 and from here to a further recirculation duct 19 the water used in the turbine 16 enters into this recirculation duct 19 and may be used to decontaminate a section 20 of seawater inside the offshore dam 17 for example if this dam bounds a port zone where it is likely that there will be a certain stagnation of the water fig4 shows a variant of the embodiment of the plant described above in this variant the offshore dam 17 has formed inside it a tank 23 which receives via the duct 11 u2032 the water drawn by the hydraulic pump 6 prior to its entry into the turbine 16 a chamber 24 is formed underneath this tank 23 and houses the turbine 16 which is connected to the alternator this chamber 24 comprises at the top a duct 25 communicating with the tank 23 so as to.