Dystopian, goddess, special water

-The book was large, close to 8 inches tall and 4-5 inches wide
-Read it in the upper United States
-The intended audience was adult
-Series, one of the covers was of a woman in a lake with her head and hands out of the water with her head and hair back and her eyes closed. The cover was black I think
-The plot was a dystopian future with two main groups of humans, the first lived in a paradise where their goddess wants them to bathe in a special pool of water that kept their skin normal. Second group rejects the goddess and lives apart from the first but without the water their skin flakes off in big chunks. They use a white paste to try and help with the effects.
-The main character is a guy, has a girlfriend or special girl friend, and a best friend.
-The best friend switches between the two groups and feeds information to the outcast group.
-The main character has a journey and has to cross a special river but almost dies because of the effects of withdrawal from the water.