A customizable and reusable mouthpiece for diving 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 is described. The back member provides a hollow cavity portion located in the bitewing section that may be partially filled with a gel-like or other suitable substance 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 teeth. The mouthpiece requires pressure relief channels so that air or fluids may enter into and be forced out of the hollow cavity providing a bellows-like action that provides a means for the bitewing and materials of composition to return to essentially its original shape. Reusability of the mouthpiece, more specifically the bitewings is directly related to the choice and combination of materials, dimensioning and processing.