Preheat oven to 325°F. Line a roasting pan with foil, top with a shallow rack, and set aside.
Use a sharp knife to remove the rind of the ham, leaving the layer of fat intact. Score a 1-inch wide diamond pattern through the fat (no more than ¼-inch deep) and over the entire ham. Transfer to rack in the prepared pan and pour in ½ cup water.
Bake, uncovered, for 1 hour.
Meanwhile, melt butter in a medium saucepan over medium heat. Add ginger and cook until fragrant, about 30 seconds. Stir in guava nectar, honey, lime juice, dark rum, ½ teaspoon salt and black pepper to taste. Bring to a boil, reduce to a simmer and cook, stirring occasionally, until mixture is slightly syrupy, about 20 minutes. Set aside.
After ham has baked for 1 hour, brush the surface with guava glaze.
Bake for an additional 1 hour 30 minutes, brushing ham with glaze every 10 minutes. Remove from oven and let rest 15 minutes before carving.