They run large...I followed their sizing and bought an xl for a 40 inch waist and they would just fall off me if I were to dive or even swim in them.  I returned them with out a hassle and got the large size which fits much better.  It really doesn't help that they have one sizing chart saying XL is 36 and another saying XL is 40 to 42.  Neither of the listed size charts are correct.