Think of boardshorts as an addition, while swim shorts are a replacement. We wear boardshorts over our regular swimsuit or bikini bottoms for that extra level of coverage. Swim shorts, on the other hand, are specifically shorts for swimming women (Or surfing, diving, or SUPing women). Both bottoms stay put on your waist, whatever you're up to in the water. So, to review: grab boardshorts to go OVER your swimsuit bottoms; grab swim shorts INSTEAD of your swimsuit bottoms.