For this cucumber-watermelon aqua fresca, I scooped out the fruit & juice of half of a watermelon & placed it in the blender (seeds included... it would be too time consuming to pick out all the seeds). I blended it on high until it was a smooth texture. Then I strained it through a wire strainer to get the liquid separated from the pulp & seeds. Added a bit of water & about 1/2 a cucumber, thinly sliced. Give it about an hour or so for the cucumber to give its flavor to the aqua fresca. Then, enjoy! I don't add extra sugar because the fruit is naturally sweet enough for me, but you could add sugar or, preferably, simple syrup.
There you have it- the perfect summer drink! (and it's good with some vodka in it, too...)