Thaw scallops, if frozen. Cut any large scallops in half. Cook spinach according to package directions and drain really well. Keep warm.
Meanwhile, in a medium skillet, combine water, wine, salt, tarragon, pepper, and garlic. Bring to boiling and add scallops. Cover and simmer for 2 to 3 minutes or till scallops are opaque. Remove scallops from skillet with a slotted spoon. Set aside and keep warm.
In a small bowl, combine milk and flour. Stir into liquid in skillet. Cook and stir until thickened and bubbly. Add grated Parmesan cheese. Cook and stir for 1 minute longer.