Abstract: A swimming aid device in the form of a monofi monofin is contemplated. The swimming aid device may include first and second fins formed of a resilient material, each fin including an elongated internal space within a fin body, an opening leading to the elongated internal space, a fin blade coupled to and extending away from the elongated internal space, a first and second side heel connectors on opposing sides of the opening leading to the elongated internal space and sized and shaped to mate with each other, a fin blade connector and a fin blade connector receiver on the fin blade and sized and shaped to mate with each other, the combination of the first and second fins forming a monofin.