Spray 6-7 quart slow cooker with non-stick spray.
Add potato chunks and pour water over top.
Toss with salt and pepper, add garlic, and pour melted butter over everything.
Cover and cook on high for 4 to 4 1/2 hours until potatoes are completely tender.
Mash potatoes in slow cooker with all cooking liquid using potato masher.
Add sour cream and warmed heavy cream.
Use hand mixer to whip, gradually adding warmed milk until desired consistency.
Taste and adjust salt and pepper.
Serve warm with butter and chives if desired.