In a small bowl whisk together lime juice, oil, salt, cumin, and pepper; set aside.
Cut zucchini lengthwise into 1/2-inch-thick slices. Cut cauliflower crosswise into four equal slices. Cut red onion crosswise into 1/2-inch-thick slices. Brush vegetable slices with some of the oil mixture.
For a charcoal grill, grill vegetables on the rack of an uncovered grill directly over medium coals until crisp-tender, carefully turning vegetables once with a wide spatula halfway through grilling. Allow 5 to 6 minutes for zucchini and 10 to 12 minutes for cauliflower and onion. (For a gas grill, preheat grill. Reduce heat to medium. Place vegetables on grill rack over heat. Cover and grill as above.)
Meanwhile, in a small saucepan bring the water to boiling. Stir in couscous and lime peel. Remove from heat; let stand, covered, for 5 minutes. Fluff couscous with a fork.
To serve, drizzle vegetables and couscous with the remaining oil mixture. If desired, sprinkle with parsley.