Abstract: A customizable and reusable mouthpiece for scuba-diving and snorkeling use having a passageway-forming front member extending from a proximal in-mouth end to a distal outside end and a U-shaped formable back member. The back member provides an insert portion located in the bitewing section configured to provide custom moldability at substantially all points of contact between the mouthpiece and the teeth such that the mouthpiece is securely gripped by the diver's teeth. The mouthpiece may be composed of one or more materials so that at least one material will conform to the individual diver's mouth and also allow for reuse such that the material will return to essentially its original shape. Materials to provide such a reusable mouthpiece may include the use of; cellular or foamed thermoplastics, co-polymers, blends, and composites, including nanocomposites that comprise inorganics such as clays and fullerenes, and nanotubes of fullerene based derivatives.