Heat vegetable oil in a large nonstick skillet over medium-high heat. Add the onion and curry powder; sauté 4 minutes. Add coconut milk and the next 7 ingredients (coconut milk through clam juice). Bring to a boil; reduce heat, and simmer 3 minutes. Add fish; cover and cook 4 minutes. Arrange watercress evenly over fish; cover and cook 4 minutes or until fish flakes easily with a fork.