In a small saucepan stir cornstarch into water to dissolve and stir in remaining marinade ingredients. Bring mixture to a boil, whisking constantly, and simmer, whisking, 1 minute. Cool marinade for 5 minutes.
Rinse scallops and drain. In a bowl toss scallops with half of marinade and chill, covered, at least 1 hour or overnight. Reserve remaining marinade.
Wash broccoli rabe and drain well in a colander.
Heat wok or a deep 12-inch skillet over high heat until hot and add oil. Drain scallops and discard used marinade. Heat oil until it begins to smoke and stir-fry drained scallops for 1 to 2 minutes until just cooked. Remove scallops to a plate and keep warm.
Add bell peppers to wok and stir-fry 1 minute. Add broccoli rabe and stir-fry until wilted slightly and bright green, about 1 minute. Add water, tightly cover, and cook until greens are tender, 2 to 3 minutes. Add reserved marinade, stirring, until vegetables are coated well and sauce has thickened. Transfer vegetables to a platter and top with scallops.