In Grapes of Wrath, the character Noah takes a metaphorical baptism in the river. Many bathe in this river before they head through the deserts west. This cleans them physically and symbolically too. Noah feels unwanted and like he doesn't fit in with his family. After his bath in the river, he sees what he must do, and after this scene, he leaves the family and goes out on his own. This scene alludes to the character Noah from the Old Testament. Noah's connection with water and being saved through the water in this scene. 

In his book, How to Read Literature like a Professor, Thomas Foster has a whole chapter dedicated to the idea of baptism in literature and how it changes character motivation, perspective, beliefs, etc.  It has some great ideas and textual references to compelling baptism scenes. 

In The Awakening by Kate Chopin, the main character, Edna Pontieller, learns how to swim. The scene where she realizes that she has the power to enter the water and swim could be considered a baptism because she realizes her personal power. She learns to survive on her own in a situation where she is free from the constraints of the land. This knowledge becomes symbolic of her subsequent change in attitude about her role in society and her responsibilities to her husband and family. She lives in way to be free from them as well. Personal growth in water is the gateway to personal growth in all aspects of her life.

This is a good question. The most obvious example would be the baptism of Jesus. We can read of the baptism of Jesus in Matthew, Mark and Luke.

The baptism of Jesus takes place during the time John is baptizing people at the Jordan. John was baptizing people for repentance. However, the only person that did not have to repent is Jesus, because Jesus is the only one without sin according to the gospel writers. This is why John felt odd in baptizing Jesus. Jesus allows it, because Jesus is associating with his people, as sin bearer.

After this baptism, Jesus entered into his public ministry. He "began" his work as the savior of the world, according to the gospels. From this perspective, there was a transformation of epic proportions.