Near the end of the chapter, have Lina and Fia get up the feather. Return to Elendia, and talk to the fairy inside Lina's and Fia's place to find out where they are. Go to Undine Spring. Continue to the next screen, but do not talk to the fairy. A command mini-game will start. You will have only one chance to successfully complete the game to be allowed though to view the bath scene.
At the middle of the chapter there will be a type of bush. Examine and pick it up to learn that it is mint. Return to Elendia, and talk to the fairy inside Lina's and Fia's place. Go to the Undine Spring. A golem will be guarding it. Defeat it to view the bath scene.