Soaking the onions makes for a milder flavor.
Place onion in a bowl of ice water. Let onion stand 10 minutes, then drain.
Thinly slice cucumbers lengthwise with a handheld slicer or peeler and place in a large bowl. Add onion and coconut and gently toss to combine. Add coconut water, lime juice, sugar, and 1 1/2 teaspoons salt and gently toss again to combine. Refrigerate until chilled, at least 30 minutes and up to 4 hours. Before serving, season with salt and sugar.