Add the diced potatoes, ham, and onion to the slow cooker.
Pour in the water and drop in the bouillon cubes. Stir once to help the cubes begin to dissolve.
Cover and cook on low for 6 hours or until the potatoes are tender.
In a small bowl, whisk the flour into the milk until smooth. Stir this mixture into the slow cooker.
Add the butter, garlic powder, and thyme. Cover and cook on high for 20 more minutes, stirring once halfway through, until the soup thickens.
Taste and add salt and pepper as needed before serving.