Slice each cucumber in ¼-inch slices and add to a large bowl.
Add in the sliced red onion (sliced into half moons) and toss to combine.
In a mason jar or small bowl, whisk together the apple cider vinegar, water, sugar, salt, and pepper.
Pour the dressing mixture over the cucumber and onion and toss to fully coat/combine.
Keep in the fridge until ready to serve.
