21 the means 40 comprise an anchoring point 25 on the sea bottom for instance in the form of a sea anchor or like arrangement a line 26 which is coupled between the anchoring point 25 and the fin 10 and the length of which determines the swinging area of the fin 10 and thus the angle positioning area relative to the anchoring arrangement formed by the anchoring points 21 fig4 illustrates an alternative embodiment of the invention comprising a vertical post block 31 stationarily mounted on the sea bottom and a generally u shaped element 32 which is mounted centrally on the outwardly facing side of the fin 10 the post block 31 is received in the area defined by the u shaped element 32 and the fin 10 the u shaped element 32 is symmetrical relative to a central vertical plane and has its deepest part in its symmetry plane wherein the u shaped element 32 is constructed in general so as to give the fin a larger angle of inclination relative to the coastline 1 than the angle between the incoming waves 2 and the coastline 1 so that the u shaped element 32 in co act ion with the post 31 will cause the fin 10 to operate in generally the same way as in the embodiment according to fig1 in order that respective corner regions 321 of the generally u shaped element 32 shall be displaced into contact with the post 31 such that the fin 10 will give rise to the.