In a pot, sauté onions and butter; add potatoes, water, milk, salt, and pepper. Cook until potatoes are softened, then put into a crockpot; add corn. In microwave, cook celery and garlic. Add to crockpot. Put fish into a small dish and microwave for 1 minute. Add to crockpot. Add thyme, wine, and parsley, if desired. Cook on high for 2 hours.