a swimming aid device in the form of a monofi 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 .