Heat olive oil in a pot over medium heat and cook onion and garlic until soft and translucent, about 5 minutes. Add zucchini; cook and stir for about 5 minutes. Pour in water and add salt; bring to a boil. Partially cover pot, reduce heat, and simmer until zucchini are soft, about 15 minutes.
Add basil and puree soup using a stick blender until smooth. Season with salt and pepper.
