This invention discloses a swimming /
scuba diving fin (1) having a fin plate (2), a shoe (3) with an opening at the rear, an adjustable elastic band (4) that engages around the
heel of a scuba diver, and two buckles (400A) connected at opposite ends (40A) of the adjustable elastic band (4) to two opposite sidewalls of the shoe (3), wherein at least one of the buckles (400A) has a shaped hollow body (40) integrally formed and connected to a portion of the sidewall. 1) A channel (420) for axial sliding of the adjustable elastic band (4) and a releasable fastening clamp (430) for the adjustable elastic band (4), wherein the axial sliding channel (420) has a hole (403) for the inlet of the adjustable elastic band (4), a hole (404) for the outlet of the adjustable elastic band (4), and a
deflection angle (405) of the adjustable elastic band (4) positioned between the inlet hole (403) and the outlet hole (404).