If using thin-skinned cucumbers, cut in chunks. If using thick-skinned ones, like Kirbys, peel and seed, then cut in chunks.
03Working in batches if necessary, combine cucumbers, watermelon rind, vinegar and salt in the processor. Purée until smooth. Add to the other purée and whisk together well. Taste, adding salt and vinegar as needed. For a smoother texture, purée in a blender, in batches.04Chill until very cold, at least 2 hours or overnight. Taste for salt again before serving.