Combine all of the ingredients in a blender and blend until smooth. (Note: If you don't have a high-speed blender, I recommend blending the water and spinach first, to help break down the greens, then add in the rest of the ingredients and blend again.)
Serve right away for the best taste and texture. Leftover smoothie can be stored in an airtight container in the fridge for up to 24 hours, but the flavor will change as it sits.