Thaw shrimp or scallops, if frozen. If using shrimp, peel and devein shrimp, leaving tails intact . If using scallops, cut any large scallops in half. Rinse shrimp or scallops; pat dry with paper towels. Set aside. In a small bowl combine water, soy sauce, vinegar, cornstarch, and sugar; set aside.
Heat oil in a wok or a12-inch skillet over medium-high heat. (Add more oil as necessary during cooking.) Cook and stir garlic in the hot oil for 15 seconds. Add broccoli, carrot, and onion. Cook and stir for 3 minutes. Add mushrooms; cook and stir for 1 to 2 minutes more or until vegetables are crisp-tender. Remove vegetables from wok with slotted spoon. Stir soy sauce mixture. Add to wok; cook and stir until slightly thickened and bubbly. Add shrimp or scallops; cook about 3 minutes or until shrimp or scallops are opaque. Stir in vegetables; heat through. Serve with rice or pasta and sprinkle with cashews.