Combine split peas, water, onions, celery, and ham bone in soup kettle. Bring almost to a boil, reduce heat, and simmer gently, partially covered, 2-1/2 hours. Remove bone and press mixture through food mill. Return to kettle, add cream, and heat slowly. Chop meat from ham bone and return to kettle. Season with salt and pepper. Garnish with croutons.