Cut off and discard any skin from salt pork, then cut salt pork into 1/3-inch pieces. Cook in a 3-quart heavy saucepan over moderate heat, stirring occasionally, until most of fat is rendered and pork is golden and crisp, 6 to 8 minutes. Transfer pork with a slotted spoon to paper towels to drain. Pour off all but about 1 tablespoon fat from saucepan, then cook onion in fat over moderate heat, stirring, until softened, about 4 minutes. Add garlic and cook, stirring, 1 minute. Stir in peas and water and simmer, uncovered, until tender, 30 to 35 minutes. Stir in scallions and salt and pepper to taste. Serve peas over rice and sprinkle with salt pork.