Place meatballs in the slow cooker in an even layer.Whisk soy sauce, water, brown sugar, garlic, and ginger in a bowl.Pour sauce over meatballs and gently toss to coat.Cook on low for 4 to 5 hours or on high for 2 to 3 hours.Mix cornstarch with cold water and add during the last 20 to 30 minutes.Stir gently and cook uncovered until the sauce thickens.Serve hot over rice, noodles, or as an appetizer.