Cut the tuna in one inch cubes and refrigerate. In a food processor fitted with a steel blade, combine the cumin, paprika, turmeric, cayenne, garlic, onion, salt, pepper, parsley, lemon juice, olive oil and water. Process until smooth. Pour mixture over tuna and refrigerate for two hours.
Start charcoal grill. Divide the tuna into 12 parts and thread onto the skewers. Place on platter until ready to grill (up to 4 hours, if refrigerated). Reserve marinade.
Grill the tuna brochettes over a medium-hot fire for 5-8 minutes, turning every two minutes and brushing with the reserved marinade. When cooked to desired doneness, remove skewers to platter and garnish with lemon wedges. Serve immediately. 6 servings.