In 1720, François Lemoyne was the teacher of Boucher, but only for three months. Baigneuse (known in English as Woman Bathing or The Bather and her Maid), was painted after Lemoyne’s trip to Italy with François Berger in 1723/24.
This was probably one of the first Baigneuses, a bathing woman wnot based on a mythological or scripture motif. I haven’t found an older one yet. You can see a few more paintings by Lemoyne (and more baigneuses) on my tumblr.