Melt butter in a skillet over medium heat. Cook and stir onion in hot butter until tender, about 5 minutes. Add zucchini and garlic; continue to cook and stir another 2 to 3 minutes.
Pour vegetable broth and water into the skillet; bring to a boil, reduce heat to low, and cook mixture at a simmer for 10 minutes. Stir cream into the broth mixture; season with pepper. Cook until heated through, 2 to 3 minutes.