1. Position rack in bottom third of oven and preheat to 325°F. Whisk jelly, mustard, and ginger in heavy medium saucepan over medium heat until melted, about 1 minute. Remove glaze from heat.
2. Trim any tough rind and fat from upper side of ham, leaving 1/4-inch-thick layer of fat. Using long sharp knife, score fat in 1-inch-wide diamond pattern. Place ham in roasting pan. Bake 1 hour. Brush top and sides of ham generously with some of glaze. Bake until thermometer inserted into thickest part of ham registers 140°F, brushing with glaze every 15 minutes and tenting ham with foil if browning too quickly, about 1 hour 15 minutes. Remove ham from oven; let stand 15 minutes. Transfer to platter. Garnish with watercress. Serve with Rhubarb Chutney.