In a small bowl, whisk together the water, cinnamon and marmalade until well blended, then brush the mixture onto each pluot half. Place the pluots, flesh side down, on the hot grill and cook for 3 to 4 minutes, brushing occasionally with more marmalade. Turn the pluots skin side down, brush the flesh with marmalade and grill 2 more minutes until tender and caramelized.