Preheat the broiler. In a large saucepan combine meatballs, pizza sauce, and water. Heat until sauce bubbles. Cover and cook over medium-low heat for 10 minutes or until heated through, stirring occasionally.
Meanwhile, cut a thin slice from the tops of rolls; hollow out roll leaving a 1/4- to 1/2-inch-thick shell. (Discard or save bread from rolls for another use.) Place hollowed-out rolls and roll tops, cut sides up, on a baking sheet. Broil 3 to 4 inches from the heat for 1 to 2 minutes or until lightly toasted. Remove roll tops from baking sheet.
Spoon meatballs and sauce into toasted rolls. Top with cheese. Broil about 1 minute more or until cheese melts. To serve, place basil or spinach leaves on top of cheese and replace roll tops.