Chop the tuna finely. Mix with the water, salt and pepper and garlic powder. Form into 4 patties. Pack well and coat with oil. Grill 4 inches from a hot fire for 3 to 4 minutes per side, until lightly browned on both sides but still pink in the center. For the last 2 minutes of grilling, place the buns, cut sides down, on the grill to toast the interior surfaces lightly. Serve the burgers on the toasted buns. Per serving: 305 calories, 20 g carbohydrates, 12 g fat, 70 mg cholesterol, 670 mg sodium.