In a medium saucepan melt butter over low heat. Stir in peanut butter until thoroughly combined. In a small bowl stir together syrup and the water; stir into peanut butter mixture. Bring to boiling over medium-high heat, stirring constantly. Reduce heat. Simmer, uncovered, about 5 minutes or until slightly thickened. Serve warm or cool. Cover and refrigerate the remaining syrup for up to 3 days if made with chunky peanut butter or up to 1 week if made with creamy peanut butter.