Shounen Teikoku

Shounen Teikoku (translated as Boy's Empire in early chapters or Boys' Empire in later ones) is a manga series that began in 2004 by Gotou Kenji, who also goes by Juan Gotoh. There are around nine to eleven chapters in total.

There are bath scenes involving the central protagonist named Tamura Makoto (nick-named Maa-kun) sharing baths with female characters.

Chapter 2
In part 1 of chapter 2, Makoto takes a bath with his older sister, Tamura Mina:

Chapter 3
In part 2 of chapter 3, Makoto and Mina's mother, Tamura Umeko, treat them to a hot spring visit. Makoto's girlfriend, Mizuhara Hitomi is also invited.

Chapter 6
In part 2 of chapter 6, Makoto's male friend Yamaguchi Ikki and Hitomi's 2 female friends Motoki Arisa (short black hair) and Hino Kotomi (long lighter hair with glasses) go Skinny Dipping in the water playing with an inflated beach ball while Makoto is applying protective SPF lotion to Hitomi.