Combine water, Scotch bonnet or habanero chiles, green onions, thyme, crushed garlic, salt, bay leaf, and allspice in heavy large saucepan. Bring to boil. Reduce heat; cover and simmer 10 minutes to blend flavors.
Add shrimp and bring to boil. Remove pan from heat; let stand until cooked through, about 20 minutes. Drain, discarding liquid. Transfer shrimp to bowl and serve warm or at room temperature, allowing diners to peel their own shrimp.