There are many ways to make fish balls, which should be the consistency of uncooked meat loaf before they are added to the hot liquid.
In soup kettle combine fish, water, carrot, celery, bay leaf, and parsley. Simmer, partially covered, 15 minutes. Strain. Remove fish and chop or shred into small bits, discarding any bones. Discard vegetables. In bowl combine fish with bread crumbs, egg, butter, and onion. Add a bit of milk if mixture seems dry. Using your hands, shape into 1-inch balls. Add fish balls and potatoes to kettle with strained broth. Simmer, partially covered, 15 minutes or until potatoes are tender. Add cream and season with salt and pepper. Heat slowly. Do not boil.