Preheat oven to 350°F. Place squash cut sides down in a baking dish. Pour water into dish around squash halves. Bake until tender and easily pierced with a fork, about 1 1/2 hours. Add more hot water if needed. Remove from oven and carefully remove skin. Set squash aside.
Boil potato cubes in salted water until very soft when pierced with a knife.
Drain potatoes and add to squash along with Red Pepper Jelly, add salt and pepper to taste.
Mash by hand until almost smooth, or use an electric mixer. Add butter if desired and serve.