Ash Ketchum

Ash Ketchum (サトシ Satoshi) is the main protagonist in the Pokemon anime.

Episode 31
Ash bathes with Misty and Brock in a Hot Spring while wearing Swimsuits.

Episode 129
He takes a bath in the 129th episode of the anime (itled "Ignorance is Blissey". While bathing, a Blissey, offered to scrub his back. Ash agrees, but she uses a rake which cuts his back.

The Electric Tale of Pikachu
In The Electric Tale of Pikachu, Ash and Brock join Misty in a Hot Spring but since she is embarrassed from bathing naked with them, she ran away.