1. To make dressing, combine 3 tablespoons olive oil, lemon zest, lemon juice, dill, 2 teaspoons minced garlic, 1/2 teaspoon salt and 1/4 teaspoon pepper in a small jar; shake to combine. Transfer 2 tablespoons dressing to a medium bowl and stir in 1 tablespoon cold water. Add avocado and toss to combine.
2. Preheat a grill to high heat or the broiler with the rack 4 inches from heat. Alternate scallops and squash on eight 12-inch skewers (if using bamboo skewers, soak them first in cold water to prevent burning). Lightly brush kebabs with half the remaining dressing.
3. Grill or broil 1 to 2 minutes, or until scallops are opaque. Turn kebabs, brush with remaining dressing, and grill or broil 1 to 2 minutes more, or until scallops are just cooked through. Serve with avocado relish.