Put the cucumber slices in a colander set over a bowl and sprinkle the salt over them. Mix well. Allow to sit 20-25 minutes. Rinse off salt, drain well and pat dry.
Slice the onion very thin and mix with the cucumbers. Heat sugar, vinegar and water, stirring, until sugar has dissolved and pour over the cucumbers. Let stand, covered, in the refrigerator for at least 2 hours to blend flavors.