A customizable scuba-diving mouthpiece for gripping with a diver's teeth, the mouthpiece including including a front member forming a horizontal passageway and a U-shaped back member with a pair of leg portions each including a thermoformable custom-moldable bite portion which in some embodiments is substantially X-shaped in cross-section along the bite-line. Upper-inner and lower-inner wall-portions of each leg portion may terminate rearwardly beside the diver's premolars and forward of the diver's first molar, thereby reducing gagging effect. The passageway may be defined by tapered inner sidewall portions defining the greatest between-sidewall-portions cross-dimension at the leg portions. A bite-limiter may be inserted into the passageway and includes a bite-limiting portion positioned for engagement with the diver's incisors to control the degree of bite and depth of tooth impressions, the bite-limiting portion having a thickness no greater than one third of a passageway vertical dimension.
