Place ham in a large roasting pan with bone sticking up. Pour mustard over the ham and, using your hands, rub it into crevices. Pour molasses over top; add water to the roaster. Sprinkle with salt and pepper.
Roast in 350°F (180°C) oven for 2 hours, then reduce heat to 250°F (130°C) for an additional hour.
Turn the pan in the oven every hour or so; baste ham with juices every 20 to 30 minutes, using a turkey baster.
Remove ham from oven and let cool. Continue to baste ham for a few hours, if you wish.