The invention relates to the new structure of the fin or the fin's blade (2). The blade (2) is made of segments (S) and transitions (T), wherein a segment with a positive incline and a segment with a negative incline alternate along the blade (2). For example, two neighbouring segments (S) with a transition (T) can form the shape of a wave, a triangle, a trapezium or a tooth. Individual segments (S) can be flat, mainly flat or curved. The lengths of two neighbouring segments (S) with a transition (T) define total length (L). Heights (H) of segments (S), total length (L) and transition (T) length can be equal, they can increase or decrease linearly, progressively or regressively. Random combinations of changing shapes, heights (H), segments (S), total lengths (L) and transitions (T) along the blade (2) are possible. Segments can follow each other across the entire width of the blade's (2) surface or optionally in one part of the fin. Preferably, segments (S) are produced in the shape of waves with connective transitions, thus in the shape of a sinusoid. Preferably, the height (H1) of the segment is the highest at the root (7) of the blade (2), where the foot pocket (1) is installed, and decreases towards the ending (8) of the blade (2) until the transition to the flat part (10). Preferably, total lengths (L) are equal or increasing from the root (7) towards the ending (8) of the blade (2).