Water World: Book One

There's more to the world than Sam and Dean truly know, they only know whats on land. When on a hunt after a man claims to have caught a merman and even showed it to the world, only problem was, he looked exactly like Sam. He was taken to an aquarium for the entertainment of humans. Sam and Dean go to the location that the mer-Sam was found, finding another merman just barely above the water, watching. Apparently there is a whole other world but with merfolk, minus the monsters, demons, and angels. Now it's up to them to save the lost mermaid with the help of two normal people, an angel and an archangel