Because it's a day of relaxation and bonding~ You'll get to know more about the people here and that will help incorporate things into your plan. It sure is nice to get to know other people, if not for camaraderie, then knowing which people should do what in an important thing like a battle against the Daitengu.

[ Also, she changed her mind. They're not going to the bath. They're going to the jacuzzi. They don't even need to change. Just dunk themselves in the warm jacuzzi with water jets blasting at them underwater.

So 45 places Jiro inside the tub and makes sure he doesn't try to run away the second she lets him go. ]