Soak lima beans in 2 quarts of water in a large bowl for 8 hours or overnight. The next day, drain the lima beans and place into a slow cooker with onions, ham bone, and ham. Pour in 3 cups of water or as needed to cover. Place lid on the cooker, set to High, and cook for 3 hours.
Stir in Cajun seasoning, black pepper, garlic salt, and cayenne pepper; set cooker to Low and cook until the beans and meat are very tender, about 4 more hours.