Peel shrimp, reserving shells; cover and chill shrimp. Combine reserved shrimp shells and water in a medium saucepan. Bring to a boil; reduce heat, and simmer 10 minutes, stirring occasionally. Strain broth through a sieve into a large bowl; discard shells.
Combine reserved broth, onions, and next 12 ingredients (onions through bay leaves) in a large saucepan. Bring to a boil; reduce heat, and simmer 10 minutes. Cool to room temperature. Combine shrimp and syrup mixture in a dish; cover and marinate in refrigerator 1 hour.
Remove shrimp from dish, reserving marinade. Place reserved marinade in a small saucepan; bring to a boil. Cook 1 minute; remove from heat.
Thread 6 shrimp onto each of 8 (12-inch) skewers. Place kebabs on a grill rack coated with cooking spray, and grill for 6 minutes, turning once. Serve with reserved marinade, if desired.
Note: Dark cane syrup (such as Steen's) may be substituted for golden cane syrup.