Rinse the beans thoroughly and place them in a 7-quart slow cooker along with the water, onion, garlic, and bay leaf. Cover and cook on LOW for about 8 hours, or until the beans are nice and tender.
Remove the bay leaf. Using a handheld immersion blender, puree the remaining ingredients to the desired texture.
Add the salt to taste.Ladle the soup into bowls.
Drizzle with the olive oil, sprinkle with rosemary, and serve.