Combine water, sugar, salt, vinegar and dill into a large bowl. While mixture is dissolving and blending, thinly slice cucumbers and add to the marinade. Thinly slice onion (if desired) and add to mixture. Chill in fridge until ready to serve. I find the longer you can let the cucumbers sit and absorb the better, but you can also enjoy right away. I also make sure to give it a good shake every once in a while. Also once you devour the cucumbers, there is plenty of marinade left to toss in some more cucumbers!