Place potatoes and water in 3-quart (large) slow cooker pot. Add butter and cream cheese.
Cover and cook on HIGH 4-to-5 hours or LOW 8-to-9 hours. When potatoes are tender, mash in slow cooker pot until smooth and creamy, using potato masher or hand mixer. Stir in 1 cup cheddar, half and half, chives, 1-1/2 teaspoons salt and 1/2 teaspoon pepper.
Top with remaining cheddar and cover. Cook on HIGH until cheese is melted, about 10 to 15 minutes.
Serve immediately or use the WARM setting to keep potatoes warm for up to 4 hours.
Provided by Meggan Hill of Culinary Hill