Heat gas or charcoal grill. In 8- or 9-inch square glass baking dish, place sweet potatoes and water. Cover with plastic wrap, folding back one edge 1/4 inch to vent steam. Microwave on High 4 to 6 minutes or just until tender; drain.
Meanwhile, in small bowl, mix remaining ingredients; set aside.
Place potatoes on grill. Cook uncovered over medium heat 3 to 5 minutes, turning frequently and brushing with butter mixture, until potatoes are tender and lightly browned.