Preheat oven to 350 degrees F. Grease two muffin tin pans with cooking spray or butter then dust with flour; set aside.
Wash zucchini and dry. Grate zucchini; set aside. Do NOT drain or remove zucchini liquid.
In a large bowl, whisk together flour, sugar, brown sugar, baking soda, salt, cinnamon, and nutmeg.
In a separate bowl, whisk together eggs. Add in vegetable oil, water, lemon juice, and vanilla extract. Add in grated zucchini then whisk the mixture.
Carefully pour 1/2 wet mixture into dry mixture. Fold in with spatula then add remaining liquid to dry mixture. Add nuts, if using. Fold mixture together until white streaks are gone.
Spoon 2 1/2 tablespoons mixture into each muffin tin. Bake in oven 18-22 minutes or until tops are set and a toothpick inserted into the center comes out clean.
To make the icing, combine water and powdered confectioners' sugar. Whisk until lump free then drizzle over the tops of the muffins with a spoon.
Allow icing to set then store in an airtight container or ziplock bag.