Part of the reason Japan has so many successful franchises, despite being such a small island nation (Pokemon and One Piece are two of the most internationally successful series of all time) is their ability to tap into the wallets and hearts of... uh... perverts. A lot of people like anime booba, even if you don't.
Pokemon literally made a fursona into a starter (Incineroar), and while One Piece isn't always the lewdest show, you best believe it's FULL of boobs.
Bath scenes have happened plenty of times in this series, and serve as a way for the crew to bond together in their most natural state (naked), while also providing fan-service to people who simp for the characters. It's going to be a few panels, at most, it's not that serious.
People on the internet stay getting WAY TOO MAD over cartoon titties.
(Edit: spelling edit)