With a grapefruit knife, cut the core of the pineapple away and discard, then cut the fruit away from the skin and cut the flesh into crosswise slices. With either bamboo or metal skewers, run a skewer from the stem end up through the flesh to the top to keep the fruit slices together. In a small pan, combine 2 Tbs. water, butter, sugar and ginger. Bring to a boil and cook until it becomes syrupy. Take off the heat and add the rum. Stir well and brush the pineapple with the mix. Grill the pineapple cut side down until warm, grill-marked and the glaze is bubbly. Remove pineapple to a plate and pour the remaining sauce over the pineapple and serve warm.