Modular and adjustable swin fin

The present invention extends to apparatuses and methods directed to a modular and adjustable swim fin. Some aspects of the invention include a swim fin having one or more interchangeable fin ribs that facilitate adjustment of flexibility characteristics of the swim fin. For example, interchangeable fin ribs having different flexibility characteristics (e.g., durometer, stiffness, rigidity, or hardness) provide desirable performance when attached to a fin blade of a swim fin. In addition, some aspects of the invention include a fin blade with an adjustable fin split that facilitates use of either a solid fin blade when the adjustable fin split is closed or a split fin blade when the adjustable fin split is open.