Heat the butter in a large pot over medium-high heat. Add the scallion whites, celery and bell pepper and cook, stirring occasionally, until the vegetables are crisp-tender, about 4 minutes. Add the flour and Old Bay Seasoning and cook, stirring, until the flour is lightly toasted, about 1 minute. Add the milk, 6 cups water and the rice. Bring to a rapid simmer, then reduce the heat to medium and cook until the rice is tender, about 15 minutes. Add the corn to the pot and cook 3 minutes. Stir in the shrimp and cook until they curl and turn opaque, about 3 more minutes. Season the chowder with salt and pepper and stir in the sherry. Ladle into bowls and top with the scallion greens.