A. We're not aware of a shapewear product specifically designed to be worn under a swimsuit in the water, but I suppose you could always wear a tummy flattener thong, provided it fits completely underneath your swimsuit. I can't think of anything more uncomfortable, though, on a hot summer day than shapewear underneath a swimsuit. Next time, opt for a tummy trimming swimsuit instead. There are so many styles available, including retro-inspired, that have built-in tummy panels and high spandex content to suck you in - you're sure to find one that suits you.