In the game there are two hidden bathing scenes.
In chapter 3, near the end, have Lina and Fia pick up the Feather. When you come back to Elendia, talk to the fairy inside Lina's & Fia's place to learn their whereabouts. Go to Undine Spring. Head to the next screen (don't talk to the fairy or it's going to mess up), and a Command Minigame will start. Enter the commands successfully to be allowed though. You only get one shot. Enjoy!
In chapter 4, in the middle of the chapter, there will be some kind of bush. Examine it and pick it up. It's Mint. Back to Elendia, talk to the fairy inside Lina's and Fia's place. Go to the Undine Spring. A golem will be guarding the place. Successfully destroy it and you will enjoy the last scene.