Rip the rosemary leaves from the stalks, leaving a few rosemary leaves on the end of each sprig. Cut the other end on a diagonal to form a sharp tip. Soak in cold water for 30 minutes. Finely chop 1 tablespoon of the rosemary leaves, reserving the rest for another dish.
Thread the fish chunks on to the soaked rosemary skewers, alternating with the prawns, allowing 2 cubes of fish and 2 prawns per skewer. Place in a shallow nonmetallic dish.
Combine the chopped rosemary, garlic, lemon rind and some salt and black pepper with the oil, pour over the kebabs and leave to marinate for 1 hour. Remove from the marinade. Cook on a hot barbecue for 2—3 minutes on each side until cooked, squeeze the lemon juice over the kebabs and serve with the lemon aïoli.