Place the split peas and water into a large soup pot over medium heat; bring to the boil for 2 minutes; set aside off the heat to cool for 1 hour.
Place the ham bone into the pot with the peas, and stir in onion, marjoram, thyme, garlic granules and salt; bring to the boil over medium heat. Reduce heat, cover the pot and simmer for 2 hours, stirring occasionally. Remove the ham bone and cut as much ham as possible from the bone; chop the ham and return it to the soup. Discard ham bone. Stir in celery. Bring soup back to a simmer and cook for 45 more minutes, stirring occasionally. Season to taste with salt.