In my opinion (swim) singlets would be sexy and comfortable, for example:
Gruff Pup Striker Singlet - Blue Blitz
GRUFFPUP.COM
Features Size Chart UNIQUE DESIGN – Make a bold statement with our unique Striker singlet, featuring cutouts in all the right places STRONG & STRETCHY:...
These, are just like the BearSkn swimwear made with polyester & spandex, so I think you can use them as swimwear, as well as underwear.