Swim briefs

"For racing, swim briefs for men and swimsuits for women often feature bicycle padding. Serious triathletes jump right onto their bikes after the swim, not taking time to change to cycling shorts. Unless you're tough, buy a padded suit. They tend not to fit or stay as snug as cycling shorts, so as crazy as it sounds you may want to do a practice ride in your padded swimsuit prior to race day, to make sure you're comfortable enough."