Combine the lemon juice, water, maple syrup, and basil leaves in a blender, and blend until the basil is completely broken down. No need to strain! (If your basil leaves are particularly large, start with just 2 leaves, and add more to taste.)
Serve over ice right away, or store it in the fridge in an airtight container for up to 3 days.
Alternatively, you can shake the ingredients together, and allow the flavors to meld in the fridge overnight. Remove the basil leaves before serving over ice.
You can replace the maple syrup with honey, if you don't need a vegan recipe. You might only need 2 1/2 tablespoons of honey in that case, since it's sweeter than maple syrup.