Usually when I see the word "Maillot" to describe a bathing suit I imagine a strapless or heart shaped bust bathing suit or one with vintage inspired details such as ruching at the waist. To me, a Maillot is synonymous with the bathing suit styles of the 1950s.
Now a days we just call it a one piece. But if you want to be a bit more fancy, you can say you're planning on sun bathing in your gorgeous maillot bathing suit instead of sunbathing in your one piece. *hehe*
Image credits: 1.] Vintage Maillot Advertisement by Catalina Swimwear, 2.] Yellow Maillot With Matching Swim Cap, 3.] Maillot Pin-up at the Beach, 4.] Modern Maillot, 5.] Vintage Inspired Maillot, 6.] Anthropologie.com