Midrashim says that Batya got tzaraat, so she use to go to the river nile to feel better after washing.
why did she get tzaraat?
The original text is in Pirkei d'Rebbi Eliezer 48:
והחביאוהו בבית שלשה חדשים, ולאחר שלשה נתנו אותו בתיבת גומא והשליכוהו על שפת היאור, והכל צפוי לפני הב"ה, והיתה בת פרעה מונגעת בנגעים קשים ולא היתה יכולה לרחוץ בחמין, באת לרחוץ ביאורה וראתה את הנער בוכה, ושלחה ידה והחזיקה בו ונתרפאת.
And [Yocheved] hid [Moshe] in the house for three months, and after three, she placed him in a wicker basket and sent it on the side of the river. All is forseen by Hashem, and the daughter of Paroh had been stricken with harsh plagues, and she was unable to bathe in hot waters. She came to bathe in the river, and she saw the child crying, and she sent out her hand and grabbed onto it, and she was healed.
The implication of the line "all is forseen by Hashem" is that Bisya was stricken with plagues (possibly Tzara'as, as Shemos Rabbah 1:23 proposes; h/t Dr. Shmuel) so that she would be forced to bathe in the river and thereby be able to save Moshe.